When I was hired for my current internship position, I was told that I would be reporting to person X - his name was listed on my offer letter, and he is who I corresponded with before my assignment started.
However, during my work I've been reporting to someone else instead, person Y.
When listing this job on my resume, should I list person X or person Y?
Don't list your managers on your resume, for any position.
Future employers will care about the work you did, and when and where you did it, not who you reported to.
For references (which you typically provide on request, at a later stage, not on your resume), use the one you actually reported to and worked with (if they'd provide a good reference, that is). Although references are typically for previous rather than current employment, since you generally don't want your current employer to know you're looking for another job, and they may not feel comfortable giving a reference. But it might be okay for roles with a fixed end date, where you're looking for jobs starting after that date.
