If the entire refrigerant charge has leaked out of a system, then a(n) ____ should be used to pressurize the system to test for leaks.

Using an inert gas to pressurize the system for leak testing is appropriate because it assists in identifying any potential leaks without introducing contaminants or reacting with the refrigerant or the system components. Inert gases, such as nitrogen, are non-reactive and provide a safe means of achieving pressure within the system to locate leaks effectively.

When an inert gas is used, it can be monitored for drop in pressure, indicating a leak, and any escaping gas can be detected visually or with the proper sensing equipment. Unlike refrigerants, which are designed to operate within a closed system, an inert gas will not damage the components or impair the functionality of the air conditioning system during testing.

In contrast, alternatives like a soap-water solution may identify surface leaks but won't pressurize the system. Gas mixtures such as butane could pose a fire hazard and would not be a safe method for testing. An oxyacetylene mixture is highly reactive and poses significant safety risks. Therefore, the most suitable and safest option is the use of an inert gas.