Final answer:
The rising phase of an action potential, known as depolarization, is caused by option (A) sodium influx (Na+) into the cell, which leads to an increase in the cell's membrane potential.
Step-by-step explanation:
The rising phase (depolarization) of an action potential (AP) is caused by sodium influx option (A). During this phase, voltage-gated Na+ channels open in response to the initial stimulus, which allows Na+ ions to enter the cell. As a consequence, the membrane potential increases from its resting potential towards a more positive value, typically reaching around +30 mV.
The depolarization phase is part of a positive-feedback loop where the initial influx of sodium ions causes more sodium channels to open, leading to further sodium entry into the cell. This rapid influx of positively charged ions is why the answer to the question is A) Sodium influx. The depolarization of the membrane is crucial for the transmission of nerve impulses in neurons and the contraction of muscles in response to stimulation.