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.