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.