Which type of testing focuses on the structural aspects of the source code?

White-box testing is a type of testing that examines the internal structures or workings of an application, as opposed to its functionality. This testing method is focused on the code itself and is often referred to as code-based or glass-box testing. It involves understanding the logic behind the code, examining control flows, and validating the correctness of the implementation against the design specifications. By having insight into the code's structure, testers can create test cases that effectively cover all paths and conditions within the software, ensuring thorough validation of every aspect of the logic.

This approach is particularly important for identifying logical errors, ensuring that all code paths have been tested, and optimizing code by identifying redundant paths. It allows for targeted testing of specific functions, providing a deeper understanding of how the application behaves internally. As such, white-box testing is essential for ensuring the robustness and security of the code.

Other testing types do not primarily focus on the internal structure of the code. System testing assesses the integrated system's compliance with specified requirements, integration testing focuses on the interactions between modules, and acceptance testing evaluates whether the system meets user needs and requirements from a functional perspective.