Which document describes how an application will be tested?

The correct answer is the "Test Plans" because this document specifically outlines the approach and methodology for testing an application. It details the testing scope, strategy, resources required, schedules, activities, and specific test cases that will be executed. A well-structured test plan ensures that all functional and non-functional requirements of the application are adequately tested and documented, allowing teams to identify defects and ensure software quality before deployment.

In contrast, the System Design Document provides specifications about how the system will be designed and integrated, focusing on architecture and technical implementations rather than testing specifics. The Requirements Definition outlines the expected functionalities and performance criteria of the application, serving as a guideline for what the application should do but not how it will be tested. Meanwhile, the API Specification describes the functionalities and behaviors of an API, including endpoints and response formats, but does not detail the testing process for the application itself. Thus, the Test Plans are uniquely positioned to explain how an application will be thoroughly evaluated during its development lifecycle.