## Core Concept
The question tests the understanding of the refractory period in the context of nerve impulse conduction. A travelling nerve impulse, or action potential, is a rapid change in membrane potential that allows neurons to transmit information. The refractory period is a critical concept here, referring to the time during and after the action potential when the neuron is either unable to generate another action potential (absolute refractory period) or can only generate one with a stronger-than-normal stimulus (relative refractory period).

## Why the Correct Answer is Right
The correct answer, , implies that the area immediately behind a travelling nerve impulse does not depolarize because it is in the **absolute refractory period**. During the absolute refractory period, the sodium channels that opened to initiate the action potential are still open, but the potassium channels are also open, leading to repolarization. More importantly, the sodium channels have an inactivation gate that closes shortly after they open, making it impossible for the membrane to depolarize again until these channels are reactivated and the membrane potential returns to its resting state. This period ensures that the action potential moves in one direction and prevents the backward propagation of the impulse.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not directly relate to the physiological mechanism preventing immediate depolarization behind a travelling impulse.
- **Option B:** This option is incorrect as it suggests a reason that does not accurately describe the physiological basis for unidirectional propagation of the action potential.
- **Option C:** This option is incorrect because, although related to the refractory periods, it does not accurately capture the reason a travelling nerve impulse does not depolarize the area immediately behind it.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **refractory period** allows for the unidirectional propagation of action potentials and sets the limit on the maximum frequency of action potentials a neuron can generate. This concept is crucial for understanding how neurons integrate and transmit information.

## Correct Answer Line
**Correct Answer: D. Refractory period.**