Which of the following is a test that is done on an existing system to...

Regression testing is a type of testing that is done on an existing system to ensure that the previous functionality still works after a change has been made. Regression testing is a process of retesting or verifying that a software system or application still performs as expected after it has been modified or updated with new features, bug fixes, patches, etc. Regression testing can help to detect any errors or defects that may have been introduced by the change and ensure that no functionality has been broken or degraded.