Refractory period:
The refractory period refers to a period of time during which a nerve or muscle is unresponsive to stimuli, particularly after it has been stimulated. This period is essential for the proper functioning of nerves and muscles.

Explanation of answer:
The correct answer is option 'B': Nerve cannot transmit impulse during the refractory period. This is because during this period, the nerve is in a state of temporary inactivity and is unable to respond to any new stimuli.

Types of refractory periods:
There are two main types of refractory periods:

1. Absolute refractory period: During the absolute refractory period, the nerve or muscle is completely unresponsive to any new stimuli. This is because the voltage-gated sodium channels, which are responsible for generating action potentials, are inactivated and unable to open. As a result, no new action potentials can be generated.

2. Relative refractory period: Following the absolute refractory period, there is a relative refractory period during which the nerve or muscle can respond to a new stimulus, but only if it is stronger than usual. This is because some of the voltage-gated sodium channels have recovered from their inactivated state, allowing for the generation of new action potentials. However, the cell membrane is still hyperpolarized, requiring a stronger stimulus to depolarize it and generate an action potential.

Importance of refractory period:
The refractory period is essential for several reasons:

1. Preventing overstimulation: The refractory period ensures that nerves and muscles do not become overstimulated by preventing the generation of multiple action potentials in quick succession.

2. Maintaining directionality of nerve impulses: By preventing the backward propagation of action potentials, the refractory period ensures that nerve impulses can only travel in one direction along the nerve fiber.

3. Regulating the frequency of nerve impulses: The refractory period helps regulate the frequency at which nerve impulses can be generated. It allows for a brief recovery period before the nerve or muscle can respond to additional stimuli.

Conclusion:
During the refractory period, a nerve cannot transmit impulses. This is because the nerve is temporarily unresponsive to stimuli due to the inactivation of voltage-gated sodium channels. The refractory period is essential for the proper functioning of nerves and muscles by preventing overstimulation, maintaining directionality of nerve impulses, and regulating the frequency of nerve impulses.