Mentor .........? [closed]
Solution 1:
The preposition should be based on what type of mentorship or which aspect you wish to highlight. As such: for a field of study, "in" may be appropriate; a goal or personal/professional development target, or specific timespan would fit with "for"; "at" emphasizes the location (employer, institution, etc) over the subject of mentoring.
I might suggest eliminating the preposition altogether, also:
They are my xyz mentor.
Solution 2:
He is my mentor at the community college.
He is my mentor in mathematics.
He is my mentor from 10 to 12 this morning.