Oxygen and nitrous oxide compressed gas cylinders should not be stored:

The proper storage of compressed gas cylinders, particularly oxygen and nitrous oxide, is critical for safety due to their potential hazards. Storing these cylinders where readily combustible materials are present is particularly dangerous because both oxygen and nitrous oxide are oxidizers. Oxygen can significantly enhance combustion, potentially leading to fires or explosions if flammable materials are nearby.

In contrast, the other options provided are either safe practices or neutral to safety. Keeping the protective caps on when cylinders are not in use helps prevent damage to the valve and accidental release of gas. Storing them in a controlled temperature area, as opposed to extreme heat, is crucial since high temperatures can increase cylinder pressure, although the term "cooler than normal room temperature" may not accurately indicate unsafe conditions. Finally, securing the cylinders on a cart with a metal chain is a common practice to prevent tipping and falling, which could damage the cylinder or release gas unintentionally.

Therefore, the most critical safety principle here is to avoid storing these gases in proximity to combustible materials to prevent hazardous situations.