Dr. quillen treats a chronic smoker by first recording the number of cigarettes smoked daily, then offering monetary rewards for each day that one fewer cigarette is smoked. because the client's own behavior serves as a control, this kind of study is called a(n)

Single Participant experiment

Explanation:

Single Participant experiment also known as, Single case research design is the most commonly used  research model in the field of psychology, learning and human interactions,  where the subject acts as his or her own command instead of using another person / group.

the answer your looking for is c. : )