Final answer:
Inhibitory neurotransmitters in the CNS work by causing Cl- ion channels to open, leading to hyperpolarization and decreased neuronal firing.
Step-by-step explanation:
Inhibitory neurotransmitters of the CNS act by opening Cl- channels. When the inhibitory neurotransmitter such as GABA (gamma-aminobutyric acid) binds to its receptor on the postsynaptic neuron, it causes the opening of Cl- channels. The influx of negatively charged chloride ions (Cl-) into the neuron hyperpolarizes the membrane, making it less likely that the neuron will fire an action potential. This process has a quieting effect on the brain and is essential for the normal function of the CNS. Therefore, the correct answer to the student's question would be B) Cl-.