The refractory period refers to the period of time during which a nerve fiber is unable to transmit another impulse, immediately following the transmission of a previous impulse. This period is necessary for the nerve fiber to recover and reset before it can transmit another impulse. The duration of the refractory period varies among nerve fibers.

The correct answer to the given question is option 'B', which states that a nerve fiber in the human body, on average, transmits only 1000 impulses per second. This means that during the refractory period, the nerve fiber is unable to transmit any impulses, resulting in a reduced rate of transmission.

Below is a detailed explanation of the answer:

1. Refractory Period:
- The refractory period is the time interval during which a nerve fiber cannot be stimulated to transmit another impulse.
- After an impulse has been transmitted, the nerve fiber goes through a period of recovery and reset before it can transmit another impulse.
- This refractory period is essential for the proper functioning of the nervous system and ensures that impulses are transmitted in a controlled manner.

2. Transmission of Impulses:
- Nerve fibers transmit impulses by a process called action potential.
- An action potential is a rapid change in the electrical potential of a nerve fiber membrane, which allows the transmission of signals.
- During an action potential, the nerve fiber depolarizes, generating an electrical impulse that travels along the fiber.

3. Duration of the Refractory Period:
- The refractory period can be divided into two phases: the absolute refractory period and the relative refractory period.
- The absolute refractory period is the initial phase during which the nerve fiber is completely unresponsive to any stimulus.
- The relative refractory period follows the absolute refractory period and is characterized by a reduced sensitivity to stimuli.
- The duration of the refractory period varies depending on the type of nerve fiber and the intensity of the previous impulse.

4. Impulse Transmission Rate:
- Due to the refractory period, a nerve fiber is unable to transmit impulses continuously at its maximum potential rate.
- The average rate at which a nerve fiber can transmit impulses, taking into account the refractory period, is approximately 1000 impulses per second.
- This rate may vary depending on factors such as the type of nerve fiber and the level of stimulation.

In conclusion, the refractory period in a nerve fiber limits the rate at which impulses can be transmitted. On average, a nerve fiber in the human body transmits approximately 1000 impulses per second, considering the recovery and reset time required during the refractory period.

C option is correct