Final answer:
During the falling phase (repolarization) of an action potential, the correct answer is B) Potassium efflux, where potassium ions exit the cell.
Step-by-step explanation:
The question pertains to the falling phase, also known as repolarization, of an action potential (AP) in nerve cells. During this phase, the correct answer is B) Potassium efflux, which means that potassium ions (K⁺) leave the cell. This efflux of K⁺ is critical for restoring the negative charge inside the cell, bringing the membrane potential back towards the resting state after the peak of the action potential. The repolarization phase follows the depolarization and plateau phases.
During depolarization, sodium ions (Na⁺) rapidly enter the cell, and during the plateau, calcium ions (Ca²⁺) may also enter. However, when the membrane potential starts to return to its resting state, the potassium channels open, allowing K⁺ to exit the cell, which leads to repolarization.