The first sentence is extremely important here. (A technician is upgrading a faulty motherboard that was ORDERED by an END USER for an old, high-end desktop computer.)
If the motherboard was ordered by an end-user, it's possible that the end-user ordered the incorrect one, it would indeed be prudent for the technician to consider verifying the socket type and manufacturer, as mentioned in option C. This step can help confirm whether the motherboard is compatible with the CPU.
So, to clarify, the technician should initially verify the socket type and manufacturer (option C) to determine if there is a compatibility issue. If a compatibility issue is confirmed, then finding a compatible motherboard for the CPU (option D) or sending this motherboard back to the manufacturer for the correct motherboard and socket would be the appropriate next step.