Incumbent means necessary for someone as a duty or responsibility.
Option C is the correct answer because “binding” means an agreement involving an obligation that cannot be broken.
Hence option C is the most apt meaning for the given word.
Option A is incorrect because “pointless” means something that has no relevance, is not related to the given word.
Option B is incorrect because “depending” means be controlled or determined by, hence this is incorrect because it is not the exact meaning of incumbent.
Option D is incorrect because “optional” means something that is a choice given for one to make with full freedom without any force, which is incorrect here because it is irrelevant to the given word.