The correct answer is: compulsory.
"Incumbent" means "required or expected as a duty or obligation." In this case, it is saying that it was the man's duty to report the matter at once.
"Pointless" means "having no purpose or use." This is not the correct answer because it is not saying that the man's duty was pointless.
"Depending" means "subject to the condition or action of something else." This is not the correct answer because it is not saying that the man's duty was dependent on something else.
"Optional" means "not required or necessary." This is not the correct answer because it is saying that the man's duty was not optional.
