Which characteristic defines white box testing?

White box testing is characterized by its focus on the internal structure, design, and implementation of the software being tested. It specifically involves an understanding of how the code functions, which allows testers to create test cases based on the internal workings of the application. This type of testing enables comprehensive coverage of the application's control flow, data flow, and logic, facilitating the identification of hidden errors and improving overall software quality.

While the other options mention important aspects of testing, they do not capture the essence of white box testing. For example, basing testing on specifications or test basis documentation is more aligned with black box testing, where the focus is on input-output without considering internal mechanisms. Including both functional and non-functional testing does not exclusively define white box testing, as this approach can be applied in various testing levels and methodologies. Thus, the defining characteristic of white box testing lies in analyzing the internal structure of the system, making the correct answer a reflection of its core principle.