What is the speed of nerve impulse conduction in mammalian nerve fibers?

When a neuron is not conducting any impulse,$i.e$.,resting,the axonal membrane is comparatively more permeable to:

Repolarization of a nerve fiber is caused by which of the following?

What is a polarised nerve fiber?

Resting membrane potential is maintained by

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.