Final answer:
Clarisse McClellan's simple question to Montag is "Are you happy?", which prompts Montag to later admit to himself that the answer is 'No'.
Step-by-step explanation:
The simple question that Clarisse McClellan asked Montag which led to his internal admission that the answer is 'No' is, "Are you happy?". Montag's conversations with Clarisse described about his own life and satisfaction. This seemingly innocent inquiry by Clarisse acts as a catalyst for Montag's self-awareness and eventual transformation throughout the novel. It exposes that despite his secure job and seemingly normal life, Montag realizes he is not truly content. Clarisse's question is a pivotal moment, highlighting the theme of personal fulfillment versus societal expectation, which is a central conflict in Montag's character development.