Out of the following given choices;
A) excitatory, but not inhibitory, synapses
B) inhibitory, but not excitatory, synapses
C) electrical synapses
D) ligand-gated ion channels
The answer is D. These are transmembrane ion-channel proteins that open to allow ions such as Na, K, Ca, and/or Cl to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter.