The correct answer is option d - branch-like fiber extending in clusters from a neuron's cell body.
Axons are regions that extend from neuron cell membrane. The axon starts from a cell body portion called axon hillock. Then it extends to form a terminal towards the end that is adjacent to the target cell.
Axons are considered the primary transmission lines in the nervous system. They can be quite long. Some can extend to over a meter long, for example those of the sciatic nerve.
