John works as a systems engineer for BlueWell Inc. He has modified the software, and wants to retest the application to ensure that bugs have been fixed or not. Which of the following tests should…

Reliability testing evaluates the software's ability to perform its required functions under stated conditions for a specified period, not primarily to check if bugs were fixed or new ones introduced by changes.

Functional testing verifies that each function of the software operates according to the requirements specifications, but it is typically applied to new features or specific functionalities, not universally to ensure existing functionality remains intact after changes.

Performance testing evaluates the responsiveness, stability, scalability, and resource usage of a system under a particular workload, and is not focused on bug verification or preventing regressions.

Regression testing is specifically designed to ensure that recent changes or bug fixes to a software application have not negatively impacted existing functionalities and that the previously working parts of the system still function correctly. When John modifies software to fix bugs, he needs to perform regression testing to verify that the bug is indeed fixed and that no new bugs or regressions have been introduced elsewhere in the application due to the changes.