The correct option is C synapse
Synapse is the junction between the axon of one neuron and the dendrite of another, through which the two neurons communicate.
The nerve impulse is also called 'action potential'. It refers to the electric signal produced by a neuron when stimulated.
A joint is an articulation between two bones in the body and are broadly classified by the tissue which connects the bones.
The axon hillock is the last site in the soma where membrane potential propagated from synaptic inputs are summated before being transmitted to the axon.