According to who, is the definition of health​

To add on to what the other person said that is the WHO (world health organizations) definition of health :)

Aand b would both be correct

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.