A different way to catch this is to isolate every "one of the..." type phrase across the sentence and apply the plural rule as a checklist, rather than reading part by part for general fluency.
This checklist approach isolates the singular/plural mismatch to part (c) alone, confirming option C as the answer.
Let's summarize:
Keeping a mental checklist of common grammar traps like this one speeds up error-spotting sections considerably.
Identify the part of the sentence that contains an error:
"Neither the manager nor the employees ___ willing to accept the new policy."