Regression Testing is :
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IT is a method by which individual units of source code are tested to determine if they are fit for use.
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It is the testing of integrated system to evaluate the system's compliance with its specified requirements.
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Testing that seeks to uncover software errors by partially retesting a modified program.
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None
This is a duplicate of question 152336. Regression testing is testing performed after code modifications to verify that existing functionality remains unaffected. It selectively retests parts of the system that could be impacted by the changes.
Regression testing specifically means re-running tests against a modified program to confirm that recent changes haven't broken existing functionality — i.e., 'testing that seeks to uncover software errors by partially retesting a modified program,' which matches option 3 exactly. Option 1 describes unit testing (testing individual code units in isolation), and option 2 describes integration/system testing (validating the integrated system against requirements) — both are different testing types, not regression testing.