Final answer:
After the peak of an action potential, the membrane repolarizes, voltage-gated K+ channels open, and some voltage-gated Na+ channels inactivate, thus all of these events occur simultaneously.
Step-by-step explanation:
After the peak of the action potential, several things occur: (1) the membrane repolarizes, (2) voltage-gated K+ channels open, and (3) some voltage-gated Na+ channels inactivate. Therefore, the correct answer is (4) all of these occur.
During the action potential, when the membrane potential reaches the threshold, voltage-gated Na+ channels open rapidly, allowing Na+ ions to flood into the cell, which leads to depolarization. At the peak of the action potential, the inactivation gates of the Na+ channels close, preventing more Na+ ions from entering. Almost simultaneously, voltage-gated K+ channels open up, allowing K+ ions to leave the cell, which begins the process of repolarization. As more K+ exits, the cell's internal voltage returns to a more negative state. Eventually, the K+ channels close and the Na+/K+ pump helps restore resting potential, completing the repolarization phase and returning the membrane to its resting state.