6346. It was incumbent on him to report the matter at once.

✅ The correct answer is option C.
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