With regard to the process of neural transmission, a refractory period refers to a time interval in which:

a. a brief electrical charge travels down an axon.

b. an individual reflexively withdraws from a pain stimulus.

c. chemical messengers go across the synaptic gaps between neurons.

d. a neuron recharges before it can fire again.

Refractory periods are a short phase in time following an action potential where another action potential cannot be generated.
It is the period immediately following the transmission of an impulse in nerve or muscle, in which a neuron or muscle cell regains its ability to transmit another impulse.
There are two types of refractory period, that is the absolute refractory period and the relative refractory period. Absolute refractory period is the first part of a refractory period during which, the neuron will not fire again no matter how great the stimulation and this only lasts for a short time.
Relative refractory period occurs when a stronger than usual stimulus is required to trigger the action potential before the neuron returns to resting state.