The answer is B: fat gives your body energy to function, while muscle mass is used only to perform strenuous activity. This option is only correct because the statement includes the words, "muscle mass." If it was only "muscle," then it would be wrong.
The answer cannot be A because weight should definitely be considered when discussing overall health.
The answer cannot be C because muscle weighs less than fat, but it is healthier.
The answer is not D because body protection does not occur from the amount of fat a person has.