I need help on this question; What description matches the alternative fitness principle of recovery?

a) Consistency helps keep you motivated and your body in motion.
b) In order for a fitness program to be fully effective, exercises that address all fitness components must be incorporated on a regular basis.
c) A workout program must progress in its level or difficulty in order for the body to continue improving.
d) A hard day of intense exercising should be followed by an easier day with a lighter workout.