A student thinks quitting vaping is clearly a good idea, and he is confident he could stop if he decided to. He has no intention of stopping. On the Theory of Planned Behavior, which component best explains the gap, and why?

D. Subjective norms, which can pull hard against a positive attitude.

Intention has three feeds, and the description hands you two of them, attitude and control are both favourable. That leaves norms to account for the gap: if quitting would mark him out among the people he sees every evening, the model expects that to show up in intention.