Final answer:
Succinylcholine is the agent listed that is NOT an anesthetic inhalation agent; it is a muscle relaxant used in conjunction with general anesthesia.
Step-by-step explanation:
The non-anesthetic inhalation agent from the options provided is D. Succinylcholine. While Halothane, Enflurane, and nitric oxide are all agents used for inhalation anesthesia, Succinylcholine does not belong to this category. Instead, Succinylcholine is a depolarizing skeletal muscle relaxant used during the induction of general anesthesia or intubation, but it is not an inhalant. Volatile anesthetics such as Halothane and Enflurane have been widely replaced by newer agents like isoflurane, desflurane, and sevoflurane due to concerns regarding safety and side effects.