The correct option is A wants
The sentence is in simple present tense as it indicates an action that is a part of a plan or arrangement.
"Wants" in option A is correct as it is third person present participle of the singular verb. This is in subject verb agreement with the singular subject 'headmaster'.
"Is wanting" is an adjective that indicates lacking. This is not required here.
"Was wanting" is incorrect as it indicates the past participle of the adjective and indicates lack in something in the past.
As option A is correct, option D 'none of these' is void.