Which of the following trailers holds compressed gas at a pressure of 500 psi?

The MC331 trailer is specifically designed to transport liquefied gases under pressure and typically operates at pressures ranging from 100 to 500 psi. This makes it suitable for carrying various types of compressed gases, including propane and ammonia.

On the other hand, the MC338 trailer is intended for the transportation of cryogenic liquids, which are gases that have been cooled to a very low temperature and may also hold some gas at high pressure, typically above 500 psi. However, it is worth noting that while it can handle gases, it is more specialized compared to the MC331.

Given the nature of the question, both types of trailers can deal with pressures at or around 500 psi, which is why the correct response identifies both MC331 and MC338 as relevant options. Thus, the selection of both as correct indicates that the question recognizes the capabilities of each trailer type concerning compressed gases at that pressure.