Final answer:
An antagonist is a substance that decreases the activity of neurotransmitters by blocking their effects or interfering with their production. Therefore the correct answer is Option D.
Step-by-step explanation:
The root that means the working against the driver of an action in the context of pharmacology is antagonist. An antagonist is a type of substance that decreases the activity of neurotransmitters by blocking their receptors or interfering with their synthesis, preventing the neurotransmitters from binding to their receptors and exerting their normal effects. As opposed to agonists, which mimic the effects of neurotransmitters, antagonists inhibit these effects, contributing to a decrease in neurotransmitter action.