Final answer:
Use cases are primarily used as a test basis for System level testing to verify that the integrated software meets specified requirements through real-world scenario tests.
Step-by-step explanation:
Use cases are a test basis for System level testing. System testing is a phase where the complete and integrated software is tested to verify that it meets the specified requirements. During system testing, use cases are utilized to validate the interaction between the system and its users, ensuring that the system behaves correctly in scenarios that reflect real-world usage.
Use cases are not typically used for Unit testing, as this level of testing focuses on individual components or pieces of code. They are also not the primary basis for Load and Performance testing, which targets system behavior under specific load conditions, nor for Usability testing, which primarily assesses the user interface and user experience aspects.
While prototypes are sometimes used in various levels of testing, they are more about evaluating the design and constraints rather than serving as a test basis, as is the case with use cases for system testing.