C is Ans.
If we let A represent availability, then the simplest formula for availability is:
A = Uptime/(Uptime + Downtime)
MTBF is Mean Time Between Failures
MTTR is Mean Time To Repair
A = MTBF / (MTBF+MTTR)
So MTBF=30day=30*24=720 hr
A=720/(720+12)
=98.3%