What is the main purpose of regression testing?

The main purpose of regression testing is to ensure that recent changes in the software, such as bug fixes or enhancements, have not adversely affected the existing, functional aspects of the application. This involves checking that previously developed and tested features still perform as expected after the updates have been applied.

When a defect is fixed, regression testing serves to confirm that the fix addresses the issue effectively without introducing any new problems elsewhere in the system. Thus, while verifying that the original defect has been rectified is vital, regression testing extends beyond just this point by encompassing the examination of the entire system to detect any unintended side effects from the modifications made.

The other options focus on different aspects of software testing. Some mention functionality checks which relate more to feature testing rather than regression. Evaluating performance in a new environment pertains to non-functional testing, which isn't focused on verifying the integrity of the existing system after changes.