It was incumbent on him to report the matter at once.

A. pointless

B. depending

C. compulsory

D. optional

Answer: Option C

Solution(By Examveda Team)

Pointless : having little or no sense, use, or purpose.
Depending : be controlled or determined by.
Compulsory : required by law or a rule; obligatory.
Optional : available to be chosen but not obligatory.
Incumbent : necessary for (someone) as a duty or responsibility.

Synonym of Incumbent is Compulsory