Which of the following techniques is primarily used in TDD?

The technique primarily used in Test-Driven Development (TDD) is writing tests before writing the code. This fundamental practice of TDD involves a cycle of writing a failing test case for a new feature or function, then implementing the minimal amount of code necessary to make that test pass, and finally refactoring the code while ensuring that the tests continue to pass. This approach not only guides the development process by requiring developers to consider the requirements and design of the code upfront, but it also ensures that code is thoroughly tested from the outset.

By focusing on writing tests first, developers create a clear specification for what the code should do, which can lead to better design decisions and improved code quality. This method contrasts with other approaches where tests may be added after the code is already written, which can result in overlooked edge cases and less reliable software.