Answer:
Mr. Roury exhibited referent power over John.
Explanation:
Referent power refers to a leader's ability to influence others by provoking in them a sense of admiration for his qualities and traits. Those leaders present strong interpersonal relationship skills, earning their followers' trust and respect. As we can see in the situation described, Mr. Roury never explicitly tried to influence John to become a mathematician. John's decision resulted from the admiration he felt toward his teacher. Mr. Roury is, for that reason, a reference for John, exerting referent power over him.