What question does Clairisse ask Montag before she leaves him that causes him to begin thinking?

The question that Clairisse asks Montag before she leaves him that causes him to begin thinking is that if he is happy.

Explanation:

To make Montag aware of his own feelings, Clairisse attempts to ask Montag if he is happy, knowing full well that he would be offended by that question. Clairisse successfully puts Montag into a loop of thoughts before she leaves and tends Montag to think whether he is really happy because Clairisse believes that he is not.