Depolarization, the process by which muscle fibers are stimulated to contract, occurs when:

a. calcium ions rapidly enter the cell, facilitating contraction.
b. potassium ions escape from the cell through specialized channels.
c. cell wall permeability changes and sodium rushes into the cell.
d. cardiac muscle relaxes in response to a cellular influx of calcium.

C. cell wall permeability changes and sodium rushes into the cell.

Explanation:

During depolarization, there is a decrease in the charge difference across the membrane. The inside of the membrane becomes positive compared to the outside. During depolarization, the voltage-gated Na+ channels are opened rapidly. The electrochemical gradient favors the movement of Na+ towards the inside of the cell. The inward movement of Na+  causes the depolarizing phase of the action potential and changes the membrane potential from −55 mV to +30 mV.

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