What type of test is designed to check whether changes in code have caused previously existing functionality to fail?

The correct answer is a regression test, which is specifically created to ensure that recent changes in the codebase do not adversely affect the existing functionality of the software. When new features are added, or when bugs are fixed, regression testing re-executes previous test cases to verify that those updates have not introduced any new errors or caused previously functioning features to fail. This type of testing is crucial in maintaining the integrity of the software as it evolves.

In contrast, unit tests focus on testing individual components or functions of the code, aiming to verify that each part operates correctly in isolation. Integration tests are concerned with the interaction between different components and ensure that they work properly together, rather than checking for unintended side effects of changes. Solution design, while vital in the development process, pertains to the architecture and layout of the overall system rather than testing its functionality after changes. Thus, regression tests are essential for proving that changes don't disrupt existing behavior.