The correct answer is D. Regression testing.
Regression testing is specifically designed to determine whether program changes have unintentionally introduced new errors or caused previously working functions to fail. After a system change, regression testing retests affected and related functions to confirm that the modified application still works as expected.
ISACA's glossary defines regression analysis and testing as a software verification and validation task used to determine the extent of analysis and testing that must be repeated when changes are made to previously examined software products. It also defines regression testing as a technique used to retest earlier program abends or logical errors that occurred during the initial testing phase.
Option A is not the best answer because unit testing verifies individual program modules or components. It is useful, but it does not provide the same assurance that a change has not affected existing functionality elsewhere. Option B is not the best answer because interface testing focuses on data exchange and communication between systems or components. Option C is not the best answer because integration testing verifies that components work together, but regression testing is the specific method for detecting unintended effects of changes.
This question maps to Information Systems Acquisition, Development and Implementation because ISACA's CISA Exam Content Outline includes system development methodologies, system readiness, implementation testing, and controls throughout the system development life cycle.
References: ISACA CISA Exam Content Outline, Domain 3; ISACA Interactive Glossary, "Regression analysis and testing" and "Regression testing."