A. There is no time to test the change before it goes live, so only the best developers should do this work and should not involve testers as they slow down the process.
B. Just run the retest of the defect actually fixed.
C. Always run a full regression test of the whole system in case other parts of the system have been adversely affected.
D. Retest the changed area and then use risk assessment to decide on a reasonable subset of the whole regression test to run in case other parts of the system have been adversely affected.