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. Explanation: Step by Step Detailed Comprehensive Explanation with All CompTIA Reference: Analysis of the Problem: Degraded RAID performance and red light indicate a failed HDD in the array. Option Evaluation: