I don't agree. Interview time is limited and valuable. Sure, you could ask the candidate about anything thats not illegal to ask. But why waste time on irrelevancies? Asking about a single low grade when the candidate has an otherwise respectable GPA is just wasting a chance to ask a better question. If I saw a junior interviewer do it, I would be tempted to share some ideas of stronger questions to ask in the future. Experience: hundreds of interviews.

If it were something worth asking to figure out who is a good hire, maybe - however, it's viewed as irrelevant at most jobs, so why waste precious interviewing time asking about irrelevant grades?

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.

Depends on the relevance of the course to the job.