Final answer:
The absolute refractory period is when the voltage-gated sodium channels are open and no new stimulus can trigger an action potential, as the channels are inactivated and need to reset. Option a.
Step-by-step explanation:
The term for the period when the voltage-gated sodium channels are open and the membrane cannot respond to a new stimulus, even a strong one, is called the absolute refractory period(A). During this time, an action potential cannot be initiated because the voltage-gated Na+ channel is inactivated. This phase ensures that the action potential moves only in one direction and gives time for the neurons to reset before they can be stimulated again. It lasts about 2 milliseconds, during which the voltage-sensitive ion channels return to their resting states and the sodium-potassium ATPase works to restore Na+ and K+ gradients.