Which of the following is NOT a pressurized gas used as a transport medium for dry chemical extinguishing agents?

The choice of oxygen as the correct answer makes sense in the context of dry chemical extinguishing agents. In fire suppression systems, a pressurized gas is often used to propel dry chemical agents to extinguish fires. The gases commonly used for this purpose include nitrogen, carbon dioxide, and helium, all of which are inert and do not support combustion.

Using oxygen as a transport medium is not suitable for this role since oxygen is a key component that supports combustion and enhances fire activity. Therefore, it is counterproductive to utilize oxygen in a system designed to extinguish fires, as it would not only fail to effectively deliver the extinguishing agent but could also lead to increased fire risk. This is fundamentally why oxygen is not used in systems for transporting dry chemical extinguishing agents, making it the correct choice in this question.