Which of the following is not a common refrigerant?

The correct response identifies nitrogen oxide as not being a common refrigerant. Ammonia, carbon dioxide, and Freon are all well-established substances used in refrigeration systems.

Ammonia is frequently used in industrial refrigeration due to its efficiency and low cost, particularly in large-scale applications. Carbon dioxide is increasingly being adopted in various refrigeration systems because it is environmentally friendly and has low toxicity. Freon, a generic term for a group of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), has historically been used widely in air conditioning and refrigeration but has been phased out in many applications due to environmental concerns regarding ozone depletion.

In contrast, nitrogen oxide is primarily associated with pollution issues, particularly as an air pollutant contributing to smog and respiratory problems, rather than as a refrigerant. It is not used in refrigeration systems and thus stands out as the correct choice in this context.