The rate at which a nerve impulse travels along a nerve fiber is dependent upon the diameter of the nerve fiber.

Explanation:

Nerve impulses are electrical signals that are transmitted along the length of nerve fibers. The speed at which these impulses travel is influenced by several factors, one of which is the diameter of the nerve fiber.

Effects of nerve fiber diameter on impulse conduction:

1. Influence on resistance: The diameter of a nerve fiber affects the resistance to the flow of ions during the propagation of an action potential. Larger diameter fibers offer less resistance to the flow of ions and therefore allow for faster impulse conduction compared to smaller diameter fibers.

2. Effect on membrane resistance: The membrane resistance of a nerve fiber is inversely proportional to its diameter. A larger diameter fiber has a lower membrane resistance, which allows the action potential to spread more easily across the nerve membrane and facilitates faster impulse conduction.

3. Saltatory conduction: Nerve fibers are covered by a fatty substance called myelin, which acts as an insulating sheath. In myelinated fibers, the action potential jumps from one node of Ranvier to the next, a process known as saltatory conduction. The diameter of the nerve fiber influences the distance between the nodes of Ranvier. Larger diameter fibers have larger internodal distances, which results in faster saltatory conduction and faster impulse conduction overall.

4. Speed of ion flow: The speed at which ions can flow in and out of a nerve fiber during the propagation of an action potential is influenced by the diameter of the fiber. Larger diameter fibers have a larger surface area, which allows for faster ion flow and faster impulse conduction.

In summary, the diameter of a nerve fiber plays a crucial role in determining the speed at which a nerve impulse travels. Larger diameter fibers offer less resistance, have lower membrane resistance, facilitate faster saltatory conduction, and allow for faster ion flow, all of which contribute to faster impulse conduction. Therefore, the correct answer is option 'B' - the diameter of the nerve fiber.