What kind of unclear pronoun reference is in the following sentence Mark is a good student a helpful volunteer and a great athlete which is why he received the scholarship.?

The pronoun reference is NOT unclear. It is easy to see that the pronoun 'he' refers to the only noun for a person in the sentence 'Mark'.

An unclear pronoun reference occurs when it can't be determined which person is referred to; for example:

Brothers Mark and Jim are good students, helpful volunteers, and great athletes, which is why he received the scholarship.

Which brother received the scholarship? It's not clear.