**Core Concept**
The all-or-none law in neurons states that when a neuron reaches a certain threshold potential, it will generate an action potential (AP) of constant amplitude, regardless of the strength of the stimulus. This phenomenon is essential for the reliable transmission of signals in the nervous system.

**Why the Correct Answer is Right**
The all-or-none law is a result of the depolarization of the neuronal membrane, which reaches a critical threshold that triggers a rapid increase in the permeability of the membrane to sodium ions (Na+). This influx of Na+ ions causes a rapid depolarization of the membrane, generating an action potential. The amplitude of the action potential is determined by the difference in the resting membrane potential and the threshold potential, which is a constant value for each neuron. The strength of the stimulus above the threshold intensity does not affect the amplitude of the action potential.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the concept of the all-or-none law, which deals with the generation of action potentials, not the refractory period.
* **Option B:** This option is incorrect because it describes a property of the action potential, not the all-or-none law. The action potential is indeed a brief, all-or-none event, but this is a consequence of the all-or-none law, not the law itself.
* **Option D:** This option is incorrect because it describes the concept of graded potentials, which are generated by stimuli below the threshold intensity and do not result in an action potential.

**Clinical Pearl / High-Yield Fact**
The all-or-none law is essential for the reliable transmission of signals in the nervous system, allowing neurons to respond consistently to stimuli above the threshold intensity. This concept is critical for understanding the physiology of the nervous system and the generation of action potentials.

**Correct Answer:** C. All-or-none law