The correct option is D to
The preposition ‘to’ is frequently used along with ‘be’ to denote a reason or motive. In the given sentence the person’s motive is to be a professor. Hence ‘to’ is the suitable answer, hence option C is correct. On the other hand, even though the preposition ‘for’ is also used to speak about a reason or motive, it is always followed by a noun, unlike ‘to’ which is always followed by a verb. Therefore option B is incorrect. The other prepositions such as ‘about’ and ‘on’ are irrelevant in the given context. ‘About’ is used when we suggest something or when we speak regarding a particular subject, which is not the case in the given sentence. Thus option A is incorrect. Whereas ‘on’ is used to speak about the position of an object which is usually placed on top of a surface. Thus option D is incorrect.