Mr. Winters is a trainer who encourages his clients to lose weight. Instead of simply rewarding them when their weight declines, he is rewarding them each time they behave in ways that would reduce weight: each time they exercise, decline high-calorie foods, or even take the stairs at work. Reinforcing each time one gets closer to a desired behavior is known as

a. conforming.

b. extinguishing.

c. social loafing.

d. shaping.

e. classical conditioning.