In the diagram above, what does "c" illustrate?

The correct label for 'C' in the diagram is- Axon hillock.

The given diagram represents the structure of a neuron ( also called as nerve cell), which is considered as the fundamental unit of nervous system. Neuron is used for the transmission of signals in the form of electrical impulses throughout the body.

The structure of neuron primarily comprised of three parts that is Dendrite ( 'B' in the diagram),  cell body or soma ( A in the diagram), and axon ( region between 'C' and 'D').

Axon hillock is the last part of the cell body that acts like a specialized region as it connects cell body and axon. In other words, it is a junction between cell body and axon.

a and c are the answeres