Answer: D. Specificity
Explanation: Because it is an actual fitness principle!
The principle of specificity states that sports training should be relevant and appropriate to the sport for which the individual is training in order to produce the desired effect. Additionally, training should progress from general conditioning to specific training for the particular skills required in that sport or activity.
Essentially, specificity training means that you must perform the skill in order to get better at it. It is the principle behind that old saying, "practice makes perfect."