In Ethereum, there are two main types of transactions: Message Call and Contract Creation. These transaction types enable Ethereum to support both the execution of contracts and interactions between accounts.

Key Details:

Message Call Transactions: These transactions involve interactions between externally owned accounts (EOAs) or between EOAs and smart contracts. Message calls are used to transfer Ether or invoke functions within existing smart contracts.

Contract Creation Transactions: This transaction type is used specifically to deploy new smart contracts on the Ethereum blockchain. During a contract creation transaction, the code for the new contract is included, which the network processes and stores at a unique address.

Exclusion of User Transactions: While ''User'' refers to EOAs, it is not a type of transaction itself in Ethereum. Transactions in Ethereum are categorized based on their purpose -- either calling an existing contract (Message Call) or creating a new one (Contract Creation).

Thus, C. Message Call and D. Contract Creation are the correct answers, as they represent the two main transaction types in Ethereum.

Decentralized Applications (Dapps) are applications that run on a blockchain network and include embedded governance and business logic rules. Unlike traditional applications, Dapps are decentralized, meaning they operate on a peer-to-peer network rather than a centralized server, leveraging smart contracts to automatically enforce rules and protocols without intermediaries.

Key Details:

Characteristics of Dapps: Dapps are open-source, operate autonomously, and store data on a blockchain. They utilize smart contracts to handle various functions, from transaction processing to enforcing governance rules and executing business logic.

Smart Contracts: The embedded rules within Dapps are typically coded as smart contracts, which are self-executing contracts with the terms of the agreement directly written into lines of code. This ensures that all transactions and operations within the Dapp are transparent, immutable, and automatically enforced.

Use Cases: Dapps are commonly found in areas such as decentralized finance (DeFi), gaming, supply chain management, and social media, offering users more control and transparency compared to traditional applications.

In conclusion, Dapps (A) is the correct answer as it refers to computer programs running on a blockchain with embedded governance and business logic rules.

Bitcoin has a fixed supply limit of 21 million coins, as defined by its protocol. This cap is coded into Bitcoin's original protocol and cannot be altered without a consensus change, ensuring its scarcity and value proposition as a 'digital gold.'

Key Details:

Fixed Supply: Bitcoin's supply limit is integral to its design, as it imposes a hard cap on the total number of coins that can ever be mined. This limited supply is one of the factors that gives Bitcoin its deflationary nature and potential as a store of value.

Mining Schedule: New Bitcoins are released into circulation through mining rewards, which halve approximately every four years (in an event known as the 'halving'). This gradually reduces the issuance rate until the maximum supply is reached around the year 2140.

Difference from Other Cryptocurrencies: Unlike some cryptocurrencies that have flexible or inflationary supply models, Bitcoin's finite supply is a distinguishing feature that appeals to those concerned with long-term value preservation.

Therefore, D. 21 million is the correct answer, as it accurately reflects the total number of Bitcoin that will ever exist.

Tree Chains modify the standard blockchain structure from a linear sequence to a tree-like structure, where blocks can have multiple branches instead of forming a single sequential chain. This structure can improve scalability and enable parallel processing, as multiple chains can be validated simultaneously.

Key Details:

Tree Structure: In tree chains, blocks can have multiple child blocks, which allows transactions to be processed across several branches concurrently. This reduces bottlenecks associated with linear block validation and enhances throughput.

Benefits Over Linear Chains: Traditional blockchain models process blocks in a strict sequence. Tree chains allow for more flexibility and higher transaction throughput, as multiple blocks can be validated simultaneously across different branches.

Use Cases: This structure is advantageous for complex applications that require parallel transaction processing, such as large-scale blockchain networks or systems needing high transaction speeds.

Thus, D. Tree chains is the correct answer, as it refers to the blockchain model that diverges from a linear structure.

Non-Deterministic Wallets, also known as 'Just a Bunch of Keys' (JBOK) wallets, contain randomly generated private keys that are not derived from a single seed. In this type of wallet, each key is created independently and must be backed up individually, as there is no way to recover keys through a mnemonic seed phrase.

Key Details:

Random Key Generation: Non-Deterministic wallets generate private keys independently, without a hierarchical or sequential structure. As a result, each key is standalone, and losing a key means losing access to the corresponding funds permanently.

Backup Requirements: Since each key is unique and unrelated, Non-Deterministic wallets require separate backups for each key. This differs from Hierarchical Deterministic wallets, which can be restored using a single seed phrase.

Use Case: These wallets were more common in the early days of cryptocurrency, but they are less favored today due to the convenience and recoverability provided by deterministic wallets.

In conclusion, C. Non-Deterministic Wallets is the correct answer, as it refers to wallets that contain randomly generated private keys and are known as JBOK wallets.