When you improve the ability of your muscles to repeatedly exert themselves, you're improving your

Muscle endurance

Explanation:

The ability of muscles to exert themselves repeatedly is referred to the muscle endurance. Here, the muscles do not get fatigued and work continuously. Muscle endurance is due to ability of the muscles to obtain better supplies of oxygen and nutrients which in turn ensure that the muscles exert themselves without any fatigue.

i might be wrong but i think its a. let me know if i’m right and have a good rest of your day!