Which of the following can NOT be transported in an MC-331?

The MC-331 is a type of cargo tank specifically designed for transporting liquefied gases under pressure, and it has specific restrictions regarding the types of gases it can safely carry. Among the options listed, liquid argon is a cryogenic liquid that is typically transported in different specialized cryogenic tanks rather than in an MC-331, which is not designed for such low-temperature applications.

In contrast, the other substances listed—ethylene, hydrogen, and carbon dioxide—are gases that can be transported safely in an MC-331 because the tank's construction and pressure specifications are suitable for these materials. Ethylene is often transported as a compressed gas, hydrogen can be contained under pressure, and carbon dioxide is manageable in its gaseous state. Therefore, the ability of the MC-331 to handle pressurized gases makes liquid argon the only one of this set that cannot be transported in this type of cargo tank.