Which of the following should NOT be used to test for gas leakage?

Using matches, candles, or open flames to test for gas leakage is extremely dangerous and should never be done. Propane is highly flammable, and introducing an open flame in the vicinity of a potential gas leak creates a significant risk of fire or explosion. Proper safety protocols dictate that non-combustible methods must be used to detect gas leaks.

In contrast, other options, such as a soap and water solution, electronic leak detectors, and pressure gauges, are safe and effective methods for identifying gas leaks. A soap and water solution creates bubbles at the site of a leak, providing a visible indication of escaping gas. Electronic leak detectors are specifically designed to sense propane and other combustible gases and can alert the user to the presence of gas without risk of ignition. Pressure gauges can help verify the integrity of gas lines by showing whether the system maintains proper pressure without any drops that would indicate a leak. Therefore, it is crucial to avoid any method that poses a risk of ignition when detecting gas leaks, which is why open flames are unacceptable.