* Depolarization is the process where the membrane potential of a cell becomes less negative (more positive). This is a crucial step in nerve impulse transmission and muscle contraction.

* Sodium ions (Na+) are the primary players in depolarization. When a neuron or muscle cell receives a stimulus, sodium channels open, allowing sodium ions to rush into the cell. This influx of positively charged ions makes the inside of the cell less negative, leading to depolarization.

Let's look at the other options:

* b-potassium: Potassium ions (K+) moving out of the cell is associated with repolarization, which restores the cell to its resting potential after depolarization.

* c-chloride anions enter: Chloride ions (Cl-) are generally involved in maintaining the cell's resting potential. Their movement doesn't directly cause depolarization.

* d-calcium: Calcium ions (Ca2+) are important for muscle contraction and other cellular processes. While they play a role in nerve signaling, they are not the primary drivers of depolarization.

In summary: Depolarization is primarily driven by the influx of sodium ions into the cell.