The primary goal of Software Quality Assurance (SQA) is to ensure software quality. SQA focuses on establishing and maintaining processes and standards to produce high-quality software products. It involves activities such as quality planning, quality control, and quality improvement.

Fault Density is a key metric for software reliability and stability. A lower fault density indicates higher reliability, as it signifies that there are fewer defects or issues in the software. In contrast, higher fault density indicates a higher likelihood of reliability and stability problems, making the software less dependable.

The Open/Closed Principle (OCP) recommends that software entities (such as classes, modules, or functions) should be designed in a way that allows for extensions without modifying their source code. This promotes code stability and reusability.

Developers might choose to use the Proxy design pattern to improve system performance. The Proxy pattern allows you to create a placeholder for another object, which can help in scenarios like lazy loading, controlling access, or caching, ultimately leading to performance enhancements.

In the Agile model, Quality Assurance is often integrated into the development phase to ensure that the product evolves as per the customers requirements and standards. Agile encourages constant feedback and iterative development, so quality assurance is an ongoing process throughout the development cycle.

In this scenario, usability should be prioritized differently for various user groups. Novice users might need a more intuitive interface, while expert users may require advanced features and shortcuts. Customizing usability for different user types ensures a positive user experience for all.

An "informal" review is typically led by the author of the work product, and it is a less formal meeting compared to formal reviews such as inspections. Informal reviews are often more relaxed and collaborative, focusing on open discussions and quick feedback. They are valuable for catching minor issues and fostering teamwork.

The primary objective of system testing is to ensure that the entire software system, as a whole, functions correctly. It focuses on verifying that all components work together and meet the requirements. This testing level assesses the system's behavior and functionality, including interfaces between components.

The main objective of software evolution is to maintain and improve the existing software. This involves activities like bug fixing, performance enhancements, and adapting to changing requirements.

When using Black Box Testing in integration testing, one must consider API compatibility between different software components. Integration testing focuses on the interactions between components, and ensuring APIs work correctly is crucial.

The Prototype design pattern is used to create a copy of an existing object to enhance performance. It is particularly useful when creating new objects is more expensive than copying an existing one, such as in scenarios where complex object initialization is involved.

Caching frequently accessed data can significantly enhance API performance by reducing the load on the backend systems. This approach is especially valuable in high-traffic scenarios, as it serves cached responses to clients, reducing the need to fetch data from the backend repeatedly.