Which set of test types is most appropriate to verify the non-functional requirements of a client-server system?

The most appropriate set of test types to verify the non-functional requirements of a client-server system is the one that includes Performance, Load, and Stress testing.

Performance testing focuses on determining how a system performs in terms of responsiveness and stability under a particular workload. It assesses the speed, scalability, and reliability of the application, which are crucial for ensuring that the system meets the performance expectations set by the non-functional requirements.

Load testing evaluates how the system behaves under expected user loads. This type of testing helps identify the maximum operating capacity of the application and ensures that the system can handle the anticipated traffic without performance degradation. It is essential for assessing the robustness of the system in real-world scenarios.

Stress testing, on the other hand, involves pushing the system beyond its specified limits to determine how it behaves under extreme conditions. This type of testing is important for identifying breaking points and ensures that the system can recover gracefully from failures.

Collectively, these three testing types address essential non-functional aspects like performance, reliability, and robustness, making them the most appropriate choices for verifying non-functional requirements in a client-server system. Other combinations might include some relevant elements but would not provide the comprehensive coverage required for non-functional testing as effectively as this option does.