The correct answer would be option D, Heredity.
Heredity is the factor which is not critical in developing good physical fitness.
Explanation:
When a person inherit something, either physiological/biological or psychological from his parents or any side of parents, this is called as heredity.
Physical fitness is a process of making yourself physically fit. There are many factors that contribute in the development of good physical fitness like age, focus, behavior, environment, diet, workouts, etc, but heredity is not something which is crucial in developing good physical fitness.
No doubt heredity plays a vital role, as people may have physical structures or tendencies inherited by their parents, but this is not something which is crucial in the physical fitness development. A person can change his physical anatomy by working hard and putting a cap on his diet. This is something which is in the control of a person. Other factors that are actually crucial in this are not in a person's control like age, environment, etc.
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