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

Using matches, candles, or open flames to test for gas leakage is highly unsafe and should never be practiced. This method poses an immediate risk of ignition in the presence of flammable gas, leading to potentially explosive situations. Testing for gas leaks requires a safe approach where the risk of creating a spark or flame is eliminated.

In contrast, other methods listed, such as water, soap solution, and electronic leak detectors, are safe and effective for identifying gas leaks. Water can sometimes be used to check for visible bubbling at connections or joints where gas may be escaping; however, it's not the most common method for gas testing. Soap solutions are widely accepted because they can produce visible bubbles when gas escapes, indicating a leak. Electronic leak detectors are reliable tools that can detect the presence of gas without creating any risk of ignition.

Thus, the correct approach to identifying gas leaks involves using non-flammable methods that maintain safety while ensuring effective detection.