Performing verification and validation on safety-critical features involves several key activities aimed at ensuring that software functions correctly and safely throughout its lifecycle. One critical activity is performing software hazard analysis and determining acceptable levels of safety risks. This process involves identifying potential hazards that the software could pose and evaluating the risks associated with these hazards. By assessing these risks, teams can establish acceptable safety levels and implement measures to mitigate or eliminate these risks. This activity is fundamental in ensuring that the software meets safety requirements and standards, such as those outlined in safety-critical industry guidelines like ISO 26262 for automotive or DO-178C for aerospace.

Test-driven development (TDD) is a key practice in extreme programming (XP) that ensures the quality of code by defining the test pass criteria before any code is written. In TDD, developers write automated test cases before writing the actual code. These tests specify what the code should do and serve as a guide for development. The process follows a cycle of writing a test, running it to see it fail (since the code hasn't been written yet), writing the minimum code necessary to pass the test, and then refactoring the code while ensuring the test still passes. This approach helps ensure that code meets the required specifications from the outset and encourages writing only the necessary code to pass tests, leading to higher quality and more maintainable code. Reference for TDD practices can be found in books like 'Test-Driven Development: By Example' by Kent Beck.

Defect density is a metric that measures the number of defects relative to the size of the software component (e.g., per thousand lines of code). It is particularly useful for evaluating the effectiveness of the peer review process in removing defects. By calculating defect density before and after the peer review process, teams can assess how many defects were identified and addressed during reviews. This metric helps in understanding the quality of the code and the effectiveness of the review process in identifying and mitigating defects. Higher effectiveness in peer reviews typically results in lower defect density. Industry practices and standards, such as those from IEEE or ISO, often recommend using defect density as a key quality metric.

For a requirement stating that the user interface needs to be accessible to the visually impaired, the best-derived requirement is that the user interface has voice activation. Voice activation directly addresses the needs of visually impaired users by allowing them to interact with the software using voice commands instead of relying on visual cues. This makes the interface more accessible and aligns with accessibility standards such as the Web Content Accessibility Guidelines (WCAG) and Section 508 of the Rehabilitation Act, which mandate that technology be accessible to people with disabilities.

The waterfall lifecycle model is most appropriate to use when the requirements are well known and unlikely to change. This model follows a linear and sequential approach where each phase must be completed before the next one begins. Because it assumes stable and well-defined requirements, it allows for thorough planning and design upfront, which can lead to a more structured and predictable development process. Waterfall is less flexible in accommodating changes compared to iterative or agile models, making it suitable for projects with clear, stable requirements. This approach is detailed in traditional software engineering textbooks and standards such as the IEEE Standard for Developing Software Life Cycle Processes (IEEE 1074).