In a neuron,the absence of which of the following is observed in response to different types of stimuli?

Which of the following events occur during the repolarization of a nerve fiber?

On application of a stimulus on the axonal membrane,

Assertion $(A)$: The axonal membrane of the neuron is more permeable to sodium ion $(Na^+)$ and nearly impermeable to potassium $(K^+)$.
Reason $(R)$: In a resting state,a neuron does not conduct any impulse.

$A$ nerve impulse will travel through a nerve fibre only if the membrane suddenly becomes more permeable to

What is a polarised nerve fiber?