What do we call a molecule that inhibits or blocks a neurotransmitter's action?

The correct term for a molecule that inhibits or blocks a neurotransmitter's action is an antagonist. Antagonists work by preventing the neurotransmitter from binding to its receptor sites, effectively blocking the usual response that the neurotransmitter would trigger. This antagonistic action is crucial in pharmacology and neurobiology, as it can modulate neurotransmission and alter behavioral or physiological responses.

In contrast, an agonist is a type of molecule that mimics the action of a neurotransmitter, enhancing or facilitating its effects, rather than inhibiting it. The options stimulator and inhibitor are more general terms and do not specifically refer to the selective blocking of neurotransmitters. Thus, while those terms may suggest a reduction in activity, they lack the precise definition that antagonist provides in the context of neurotransmitter action.