Which type of testing focuses on the internal workings of an application?

White-box testing focuses on the internal workings of an application. This type of testing involves examining the program's internal logic, structure, and code, allowing testers to verify the flow of inputs through the code and ensure that the software behaves as expected based on its design and implementation. Testers have access to the underlying code, which helps them identify logical errors, security vulnerabilities, or optimization opportunities.

In contrast, black-box testing treats the software as a "black box," where testers evaluate functionality based on the requirements and specifications without any knowledge of the internal code structure. System testing assesses the entire system's compliance with the specified requirements, while acceptance testing validates the system with respect to user needs and expectations.