Step by Step Detailed Comprehensive Explanation with All CompTIA References: Analysis of the Problem: Degraded RAID performance and red light indicate a failed HDD in the array. Option Evaluation: A). HDD: Replacing the failed hard drive restores RAID performance. Correct. B). CPU: Not related to RAID performance. Incorrect. C). PSU: Power issues wouldn't cause RAID degradation. Incorrect. D). RAM: Unrelated to RAID functionality. Incorrect. Reference: RAID troubleshooting is outlined in Domain 5.3.