Final answer:
The principle of Specificity is applied when a trainer picks the best exercise to achieve a particular training effect by focusing on specific muscle groups, movements, and energy systems.
Step-by-step explanation:
When a trainer accurately identifies the best exercise to address a desired effect, they are using the principle of Specificity. This principle dictates that to improve in a particular area or skill, the training should be specific to the muscle, energy system, and movements involved in that area or skill. Applying this principle can involve exercises that increase the intensity, increase the frequency, or alter the target muscle groups to adapt the training to the specific demands of the sport or activity.