What best defines a regression test?

• A. Re-running tests that previously failed to confirm they are fixed.
• B. Running performance tests to ensure speed remains constant.
• C. Re-running previously passing tests after changes to ensure no new defects were introduced.
• D. Creating tests only for new features.

Answer: (C)

Regression testing is about ensuring that existing functionality still works after changes. When code is modified, even small updates can cause unintended side effects in other parts of the system. The best definition captures this by re-running tests that previously produced a passing result to verify that no new defects were introduced as a result of the changes. This helps maintain overall stability across the software as it evolves.

The other options describe related ideas but not the overall idea of regression testing. Re-running tests that previously failed to confirm they’re fixed focuses on verifying a specific bug fix rather than checking that unrelated parts of the software still behave correctly. Running performance tests targets speed or performance characteristics, which is a different concern from ensuring functional behavior remains intact. Creating tests only for new features misses the goal of regression testing, which is to protect existing features from regressions after any change.