Any cylinder containing a gas other than air must be:

For any cylinder containing a gas other than air, the correct requirement is that it must be clearly labeled. This is crucial for safety and proper identification of the contents within the cylinder. Labeling ensures that anyone handling or working around the cylinder is aware of what type of gas it contains, which can prevent potentially dangerous situations. It also aids in compliance with safety regulations and guidelines that govern the handling of compressed gases.

When cylinders contain different gases, they may have unique storage and handling requirements, and the labeling serves as a clear indication of these needs. Therefore, a clearly labeled cylinder reduces confusion and enhances safety during transportation, storage, and usage.

The other options, while they may relate to different aspects of cylinder handling or conditions, do not address the essential need for identification. Empty cylinders, those needing repainting, or those that must be stored in a cool place do not provide the same level of crucial information as a clear label does about the safe handling of the gas contained within.