The correct answer is D. Progression
Step-by-step explanation:
In training, progression implies you achieve a specific goal by increasing little-by-little the workload, which would help you to improve your physical conditions over time. This includes increasing time, intensity or difficulty while exercising.
This principle is the one Jerry is applying because to increase his arm strength he increases the repetitions progressively going from 4 reps on Tuesday to 5 repetitions on Thursday and 6 reps on Saturday. Also, he increases the intensity of other exercises as on Wednesday he attends a yoga class and on Friday he decides to run for 30 minutes which is a bit more demanding than the Yoga class.