Answer:
RAID controller or Motherboard ie. A and D
Explanation:
From the explaination on the question the two most likely system parts that may be affecting the server are the Raid controller and the Motherboard.
The raid controller, this is because after the other drive has been replaced, the Raid still failed, it needs to be checked properly and replaced.
The motherboard, this is because it is the core centre of all boards.