Answer:
Activity level.
Step-by-step explanation:
Metabolism is the important for the cell survival and its growth. Most of the metabolic factors does not depend upon the control of the organism and occurs itself in the body.
Some factors can be improved or controlled by the activity level. The activity level can be controlled by the individual and may affect the flexibility and may also affect the other metabolic factor. One can control its own activity level.
Thus, the correct answer is option (b).