A receptor with four transmembrane regions changes conformation as GABA binds. Which system is this process?

A. Presynaptic transporter: Presynaptic transporters are responsible for the reuptake of neurotransmitters from the synaptic cleft back into the presynaptic neuron. They function mainly to terminate neurotransmitter signaling rather than initiate it, and they do not undergo conformational changes in response to ligand binding like receptors do. B. Sodium-charged ion channel: Sodium-charged or sodium-dependent channels open primarily in response to voltage changes across the membrane, not ligand binding. They facilitate depolarization in neurons but are not directly activated by neurotransmitters such as GABA. C. Voltage-sensitive ion channel: Voltage-sensitive channels open or close based on electrical potential differences across the neuronal membrane. GABA binding triggers a chemical rather than an electrical response, so this process does not involve voltage-sensitive gating. D. Ligand-gated ion channel: This system opens in response to the direct binding of a neurotransmitter (ligand) to the receptor. The GABA-A receptor is a ligand-gated chloride ion channel that changes conformation when GABA binds, allowing ions to pass and leading to neuronal inhibition.