Final answer:
The creation of an action potential begins with the opening of sodium channels, which depolarizes the neuron's membrane.
Step-by-step explanation:
The initiation of depolarization during the creation of an action potential is marked by the opening of sodium channels. This influx of sodium ions reverses the membrane potential, initiating the rapid upward phase of the action potential, further characterized by depolarization. This process begins when the membrane potential reaches a certain threshold, often around -55 mV, causing voltage-gated sodium channels to open, which allows sodium ions (Na+) to rush into the neuron. This influx of positively charged ions causes the inside of the neuron to become more positive, thus depolarizing the membrane.
Following depolarization, repolarization occurs due to the opening of potassium channels (K+), which allows these ions to flow out of the cell. However, it's the initial opening of sodium channels that marks the beginning of an action potential. Consequently, the question provided is the A) Opening of sodium channels.