A verb is used to express an occurrence, action, or state. It is the grammatical centre of the predicate in a sentence.
An auxiliary verb is a verb which gives is used in the formation of tenses, moods, tones, etc.
Option C is correct as the sentence indicates duty and correctness.
The other options are wrong as the sentence expresses duty which is not fulfilled by the use of need and may.
The correct answer is C) Ought to