I think it was a stupid question, and tasteless to ask of Chelsea. As for it being a plant, consider this:
" UPDATE *** AP reports, "The college student who got a stinging brushback from Chelsea Clinton when he asked about the Monica Lewinsky scandal said Wednesday he's a Clinton supporter who was trying to get her to show 'what makes Hillary so strong.' "
This makes me more sure than ever it was a plant. Note how folks gushed over how poised and quick Chelsea was in her reply. I didn't think it was all that remarkable, but it would have been even less so, if she had prepared her reply in advance.