Which of the following gases is considered extremely toxic by the DOT?

The classification of gases as extremely toxic by the Department of Transportation (DOT) focuses on their potential to cause serious harm or death upon exposure. Poison gases are specifically designed or recognized for their toxic effects and can be lethal even in relatively small quantities. These gases can impair bodily functions, potentially leading to dire health consequences.

In contrast, inert gases do not have toxic properties that pose significant risks in normal uses, such as in controlled environments. Compressed air is simply air that is pressurized and does not exhibit toxicity, while carbon dioxide, although it can be hazardous in high concentrations, is not classified as extremely toxic in the same way that poison gases are. Therefore, poison gases are the correct answer, as they directly pose a significant threat to health and safety according to DOT classifications.