**Core Concept**
The all-or-none law states that a neuron will either generate an action potential or remain in a resting state, but it cannot produce an intermediate response. This fundamental principle is crucial in understanding the physiology of neuronal signaling.

**Why the Correct Answer is Right**
The all-or-none law applies specifically to the action potential, which is a rapid change in membrane potential that occurs when a neuron is stimulated above a certain threshold. During an action potential, a rapid depolarization of the membrane potential is triggered by the opening of voltage-gated sodium channels, followed by repolarization due to the closure of these channels and the opening of potassium channels. This results in a self-regenerating process that produces a consistent, all-or-none response.

**Why Each Wrong Option is Incorrect**
**Option B:** Resting potential refers to the stable, negative membrane potential of a neuron at rest, which is not related to the all-or-none law.
**Option C:** Membrane potential is a broader term that encompasses both resting and action potentials, but it does not specifically describe the all-or-none law.
**Option D:** Since the all-or-none law is indeed a principle that applies to action potentials, this option is incorrect.

**Clinical Pearl / High-Yield Fact**
The all-or-none law is a key concept in understanding the reliability and consistency of neuronal signaling, which is essential for the transmission of information within the nervous system.

**✓ Correct Answer: A. Action potential**