Violet is moderately attracted to Charles. When her parents forbid her to date Charles because of his age, Violet becomes very attracted to Charles. Violet's increased attraction to Charles is most likely due to

psychological reactance

Explanation:

Reactance is a phenomenon when an unpleasant event like rules, threats, taking away freedoms, or any other negative motivation awakes arousal and causes a person to strengthen attitude, contradict them and start being aggressive.

This amount of reactance is correlating with the level of threat to freedom, so the barrier person makes can be stronger or looser.

People usually very negatively react to the ones threatening their freedoms and start holding the ideas they had even stronger, which is exactly what is happening in the situation when Violet thinks her freedom is interrupted.