Which of the following is a type of performance testing?

Stress testing is indeed a type of performance testing, as it is designed to evaluate how a system behaves under extreme conditions or heavy loads. The primary goal of stress testing is to determine the system's breaking point, allowing testers to understand the limits of the application’s capabilities. This type of testing identifies how the software behaves when the demand exceeds the maximum capacity and can help uncover issues such as memory leaks, slow reducing performance, or system crashes under strain.

In contrast, user acceptance testing focuses on whether the software meets business requirements and is acceptable to end users, while functional testing verifies that software features and functionalities work as intended. Smoket testing, often referred to as "build verification testing," is a preliminary check to see if a build is stable enough to proceed with further testing. None of these options are specifically concerned with performance characteristics in the way that stress testing is.