Final answer:
The axon hillock, initial segment, and trigger zone are critical in initiating an action potential, as these areas contain a high concentration of voltage-gated Na+ channels where graded potentials summate to reach the threshold for action potential generation.
Step-by-step explanation:
The question asks about the role of the axon hillock, initial segment, and trigger zone in neural function. Among the provided options, the correct answer is A) Site of action potential initiation. These areas are crucial in a neuron as they are where the summation of graded potentials takes place, leading to the initiation of an action potential. The axon hillock and the initial segment contain a high density of voltage-gated Na+ channels that are responsible for the depolarizing phase of the action potential. If the sum of graded potentials reaches the threshold, an action potential is triggered in these locations.