Final answer:
Dextromethorphan is the substance among the provided options that acts as an antagonist at the NMDA receptor, blocking the neurotransmitter glutamate.
Step-by-step explanation:
The question asks which substance is an antagonist at the NMDA receptor. Among the given options, Dextromethorphan is known to act as an NMDA receptor antagonist. This means it blocks or impedes the normal activity of the neurotransmitter glutamate at the NMDA receptors. Dextromethorphan is often used in over-the-counter cough remedies and has been studied for its potential therapeutic effects on various neurological conditions.
The correct choice among the given options is Dextromethorphan (choice D).