Which of the Following Is an Example of the Reactivity

Which of the following is an example of the reactivity phenomenon?

A) A man joins a weight-loss centre, but quits after he feels he is not getting the desired results.
B) After joining a fitness club, a woman stops exercising in her home and instead walks 5 km a day by herself.
C) A man changes his brand of cigarettes after his father dies of lung cancer.
D) On parents' visiting day at a school, the children who often misbehave in class are unusually compliant and well behaved.