How are acetylene cylinders identified?

Acetylene cylinders are specifically designed and colored to easily identify their contents for safety reasons, particularly in environments where gases are handled, such as in welding and cutting operations. The correct identification involves cylinders made of steel, which are robust enough to withstand the high pressure of the gas. These cylinders are painted black and clearly labeled with the word 'ACETYLENE' to alert users to the presence of acetylene gas.

This identification system is crucial for preventing accidents, as acetylene is a highly flammable gas. By having a standard color and labeling system, it allows for quick recognition by workers, ensuring that the correct procedures are followed and minimizing the risk of mixing gas types that could lead to dangerous situations.

Considering the other options, painted red labeling 'OXYGEN' would denote oxygen cylinders, which are distinctly different and should not be confused with acetylene. Aluminum is not commonly used for acetylene cylinders due to the requirements for strength and durability under pressure. Lastly, a plastic cylinder painted green is not standard for acetylene and indicates an entirely different type of gas or substance, which could lead to serious mix-ups.