What Question does Clarisse ask that gets Montag on the defensive?

Clarisse asks Montag if he is happy.

She laughed at this. "Good night!" She started up her walk. Then she seemed to remember something and came back to look at him with wonder and curiosity. "Are you happy?" she said.

"Am I what?" he cried.

But she was gone-running in the moonlight. Her front door shut gently.

"Happy! Of all the nonsense." He stopped laughing. He put his hand into the glove-hole of his front door and let it know his touch. The front door slid open. Of course I'm happy. What does she think? I'm not? he asked the quiet rooms. He stood looking up at the ventilator grille in the hall and suddenly remembered that something lay hidden behind the grille, something that seemed to peer down at him now. He moved his eyes quickly away.