Final answer:
The d) sodium-potassium pump is the primary mechanism maintaining the negative charge during resting membrane potential by expelling more positive Na+ ions than K+ ions it takes in, resulting in a net negative charge inside the cell.
Step-by-step explanation:
The primary factor in the maintenance of a negative charge during resting membrane potential is the sodium-potassium pump. This pump helps to develop a negative charge inside the cell by moving three Na+ ions out of the cell for every two K+ ions it brings in. This creates a net negative charge within the cell because more positive charges are being expelled than taken in, which is crucial for preserving the -70 mV resting membrane potential commonly observed in cells. Leakage channels also contribute to this process by allowing Na+ to move into the cell and K+ to move out, but the key role of the sodium-potassium pump is to restore the ion balance, as it actively transports ions against their concentration gradients using ATP.