Which of the following best describes the exercise principle of specificity?
a. maintaining a challenging workout
b. establishing a regular exercise schedule
c. targeting a particular area for improvement
d. shifting the types of exercises in a workout

The answer is letter D. 

Specificity is a principle in training that generally requires a person to shift types of exercises in a workout that would be relevant to the fitness goal of an individual. The training should be aimed in developing specific areas that a person wants to improve on. If for example you want to improve cardiovascular endurance, you should be involved in aerobic activities like running. If you want to build muscles and get good toning, the specific exercise should be weight training. 

It is an important principle in exercise that should always be taken into consideration to attain fitness goals.

Ibelieve it's choice d.