Which of the following is considered the odd one out among testing techniques?

The choice identified as the odd one out is fundamentally different from the others in terms of the approach to testing. White box, glass box, and structural testing techniques all fall under the category of testing based on internal knowledge of the system. These techniques require an understanding of the code and architecture of the application being tested. They focus on how the code is structured, how different units of code interact, and they facilitate testing the internal workings of the application.

Conversely, functional testing aims to verify that the software behaves as intended according to the requirements. It is concerned with what the system does rather than how it does it. Functional testing does not require knowledge of the internal code structure and tends to focus on the external functionality from the end user's perspective.

Thus, because functional testing operates from an external viewpoint rather than an internal one, it stands apart from the other three testing techniques that are primarily concerned with the internal mechanisms of the software.