Psychology question please help me(=?

The answer is (b). I took a neuroscience cl and my notes say that "The All or None Law=The amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it." Action potential is the neural impulse. It's not (c) or (d) because they both contradict this law, and it's not (a) because the absolutel refractory period is the time that the cell cannot fire.