**Core Concept**
The magnitude of action potential in neurons is primarily determined by the density and distribution of voltage-gated sodium channels (Nav channels) along the axon. The rapid depolarization phase of the action potential is largely dependent on the influx of sodium ions (Na+) through these channels.

**Why the Correct Answer is Right**
The action potential magnitude is directly proportional to the number and density of Nav channels. When a neuron is depolarized, the voltage-gated sodium channels open, allowing an influx of sodium ions, which in turn causes the rapid depolarization phase of the action potential. The density of Nav channels is higher in the initial segment of the axon, which is the site of action potential initiation. The distribution of Nav channels along the axon determines the magnitude of action potential.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the magnitude of action potential is not primarily affected by the resting membrane potential. While the resting membrane potential does influence the excitability of the neuron, it does not directly determine the magnitude of the action potential.
* **Option B:** This option is incorrect because the magnitude of action potential is not primarily affected by the concentration of potassium ions (K+). While potassium channels play a crucial role in the repolarization phase of the action potential, they do not directly determine the magnitude of the action potential.
* **Option D:** This option is incorrect because the magnitude of action potential is not primarily affected by the myelination of the axon. While myelination can affect the conduction velocity of the action potential, it does not directly influence the magnitude of the action potential.

**Clinical Pearl / High-Yield Fact**
The density and distribution of voltage-gated sodium channels are critical determinants of the magnitude of action potential in neurons. Understanding the role of Nav channels in action potential generation is essential for grasping the underlying physiology of neuronal excitability.

**Correct Answer:** C.