Which of the following must be checked to determine a propane cylinder's suitability?

The suitability of a propane cylinder is critically tied to its valve functionality. The valve is a key component responsible for safely releasing gas and ensuring that there are no leaks during usage. If the valve is damaged, corroded, or not functioning properly, it can pose significant safety risks, including leaks or explosions. Therefore, checking the valve functionality is essential to guarantee that the cylinder can be used safely and effectively.

While visual appearance for scratches can provide some insight into potential damage, it does not guarantee that the cylinder is suitable for use, as internal issues might remain unnoticed. Similarly, evidence of previous fills could indicate that the cylinder has been used before, but this alone doesn't determine its safety or operational integrity. Thus, assessing the valve functionality stands out as the most crucial factor in establishing a propane cylinder's suitability for use.