What allows the action potential to return to a repolarized state?

There are choices for this question namely:

a. K+ leaks into cells.
b. Voltage-gated Na+ channels become inactivated.
c. Voltage-gated K+ channels become inactivated.
d. Na+ reaches equilibrium across the neural membrane and stops leaking in.
e. Voltage-gated Na+ channels close.

The correct answer is "voltage-gated Na+ channels become inactivated". In the events concerning an action potential, the first event is a stimulus that depolarize the resting membrane potential up to the threshold. When the threshold is reached there will be opening of voltage gated Na+ channels wherein sodium can enter the cell and make the membrane more positive therefore called depolarization. After depolarization, voltage gated Na+ channels become inactivated and K+ leaks out of the cell making the cell less negative hence repolarization. After which, more and more K+ ions leaks out making the membrane more negative than the resting membrane potential hence hyperpolarization. When K+ channels are inactivated, the cell membrane will eventually go back to its resting membrane potential.