What type of gas is primarily stored in an oxygen cylinder?

An oxygen cylinder is specifically designed to store and provide oxygen gas, which is essential for various applications, including medical purposes, welding, and industrial processes. The cylinder is constructed to safely contain oxygen under high pressure. Understanding the function of an oxygen cylinder is crucial for ensuring safe handling and usage, particularly because oxygen supports combustion and can enhance the flammability of other materials.

In this context, the other gases presented are not applicable to an oxygen cylinder. Propane and acetylene are both flammable gases commonly used in welding and heating applications but are stored in different types of cylinders specialized for their respective properties. Argon is an inert gas commonly used in welding and as a shielding gas; it also requires a distinct storage cylinder. Hence, oxygen is the only correct choice as it directly aligns with the intended use and storage characteristics of the cylinder in question.