With Respect to Monitoring Procedures, Which of the Following Is

With respect to monitoring procedures, which of the following is an example of the reactivity phenomenon?

A) On parents' visiting day in a classroom, the previously well-behaved children continue to show good behaviour.
B) A man quits smoking after his father dies of lung cancer.
C) After joining a fitness club, a woman stops exercising in her home and instead walks two kilometres a day by herself.
D) A man begins to eat less after joining a clinic weight-loss program in which each participant's weight is recorded weekly by a staff member.