Answer: (ii) Ensure recent changes didn't affect existing functionality

Regression testing re-runs previously passed test cases (and adds new ones) after a change — a bug fix, enhancement, or integration — to verify that the modifications have not broken existing, already-working functionality. It guards against "regressions": defects introduced in code that used to work.

Why the others are wrong:

• (i) Test a new module — testing new code alone is unit/development testing; regression testing focuses on existing behaviour after a change.
• (iii) Improve system performance — performance tuning is not the purpose of regression testing.
• (iv) Identify hardware errors — hardware faults are outside the scope of software regression testing.

So regression testing ensures recent changes didn't affect existing functionality — option (ii).