Thank you so much for your detailed explanation, SoLaTiDoberman!I think A and C make sense in this context.
さっき会ったから is usually means 私はさっき田中さんに会ったから.
A makes sense: As I saw him just now, it should be the timing that Mr Tanaka is still at school now.
B doesn't make sense: As I saw him just now, Mr Tanaka has the intention to be at school.
C does make sense: As I saw him just now, Mr Tanaka is supposed to be at school (yet).
D doesn't make sense: As I saw him just now, I've heard that Mr Tanaka is at school now.
I don't know which is the correct answer chosen by the examiner, but if I had to choose one answer, I would choose C like you. A might be a little weird, and C would be the best.