An enquiry was ordered to ascertain the actual cause of the accident.

Answer: Option A

Determine : ascertain or establish exactly by research or calculation.
Investigate : carry out a systematic or formal inquiry to discover and examine the facts of (an incident, allegation, etc.) so as to establish the truth.
Discover : find unexpectedly or during a search.
Prove : demonstrate the truth or existence of (something) by evidence or argument.
Ascertain : find (something) out for certain; make sure of.

Synonym of Ascertain is Determine