Verifying full system functionality occurs after implementing a fix. This step ensures the solution resolved the issue and that performance is back to expected levels. Comparing current throughput to baseline measurements is part of validation testing to confirm everything is within normal operational parameters.
A). Implement the solution applies the fix but does not confirm results.
C). Document findings happens at the end of troubleshooting.
D). Test the theory comes earlier, when trying to confirm the suspected cause.
References (CompTIA Network+ N10-009):
Domain: Network Troubleshooting - Troubleshooting methodology steps, baselines, verification testing.