I believe the answer is C) Requirement.
If something is a necessity, it is crucially important, as is a requirement. The following sentence will use the words interchangeably:
It is a necessity that you write your name on top of your paper; if you don’t, I cannot grade it.
It is a requirement that you write your name on top of your paper; if you don’t, I cannot grade it.
Hope this is accurate & helpful!