We're talking about the student's feelings were ______, and then you have a contrast down here. You have although, a word of contrast, and you have contrast here because it says although visibly happy, he was nonetheless ______, so we need something negative. Well, that would be either a insecure, or D anxious, but the only one that works correctly in the meaning of the sentence would be the answer D, he was ambivalent although visibly happy, he was nonetheless anxious.