When a neuron responds to a particular neurotransmitter by opening gated ion channels

The given question is incomplete. The complete question is:

Question:  When a neuron responds to a particular neurotransmitter by opening gated ion channels, the neurotransmitter is serving as which part of the signal pathway?

A) relay molecule

B) transducer

C) signal molecule

D) response molecule

C) signal molecule

Explanation:

Signaling cells produce specific chemicals or compounds called signaling molecules. These signaling molecules arrive at their target cells and bind to the cell surface or intracellular receptors. Binding of the signaling molecules to their receptors present on the target cells is followed by signal transduction and production of desired response by the target cells.  

Neurotransmitters serve as signal molecules that bind to the receptors present on the membranes of the postsynaptic neurons to bring about changes in the membrane potential. Their binding may trigger the opening of the gated ion channels resulting in a change in the membrane potential followed by generation of inhibitory or excitatory responses.