A C+ is a reasonable thing to ask about.
It's a waste of time, and thus unreasonable thing to ask about, wasting a candidate's time and the interviewer's time in revealing how little the interviewer knows about effective interviewing.
My technique was to learn the material :-)
I made the right decision, it worked for me. I didn't do any begging or ass kissing, and still had time to enjoy the other pursuits being in college afforded.
I see the same from some new graduates, begging their managers to sign off on something, they don’t seem to understand the difference between an academic grade and work being of a standard to show a customer.
Was it a bad teacher? A bad class? Someone in the family died? Bad breakup with girlfriend?
What business is it of yours? And, even worse, if I lie to you about why, can you verify it? And, even worse, I'm probably expecting you to ask me about it, so I'm completely prepared to snow you on that subject. Not only is asking me about it useless, it's a negative amount of information since you could have asked me about something else.
I'll go further, we had one professor at my alma mater that was well known to be a jerk. If your interviewer was from that school, passing this class was considered godly and failing it was considered a rite of passage.