Answer: Referent
Explanation:
Referent power is defined as power through which a leader tends to display such traits and personality that his/her followers admire these characteristics. Followers tend to follow leader as reference and want to become like him/her .
According to the question,Mr. Roury displayed referent power over John due to which John followed him as a reference to become like him .He became interested in numbers and math and thus ,owes his career to Mr. Roury.