The correct answer for the question that is being presented above is this one: "D. how long it takes for the voltage-gated Na+ channels to reactivate at the end of an action potential."
Here are the following choices:
A. how low the membrane potential drops below resting potential during the undershoot phase
B. how long it takes for the voltage-gated Na+ channels to close at the end of an action potential
C. how long it takes for the voltage-gated K+ channels to close during the undershoot phase
D. how long it takes for the voltage-gated Na+ channels to reactivate at the end of an action potential
E. how long it takes for the membrane potential to return to resting potential after the undershoot phase
