If the specific key is not available to turn on or off a natural gas supply line valve, which tool will work as well?

The adjustable wrench is effective for turning the natural gas supply line valve when a specific key is unavailable because it is designed to grip a variety of sizes of nuts and bolts. The jaws of an adjustable wrench can be widened or narrowed to fit the valve's operating rod or the valve itself, allowing for a secure grip as leverage is applied.

This tool is particularly useful in emergency situations where time is of the essence, and access to specialized tools may be limited. In contrast, the other options, while they serve specific functions in mechanical tasks, do not provide the same versatility or gripping capacity required to turn a natural gas valve. They might fit only certain sizes or shapes, making them less reliable for this specific task. Thus, for dealing with a natural gas supply line valve, the adjustable wrench stands out as the most practical tool.