The correct answer is A) Wheatstone bridge. Combustible gas indicators use a Wheatstone bridge circuit to detect the presence and measure the concentration of combustible gases. The Wheatstone bridge is a sensitive electrical circuit that can detect small changes in resistance, which is the principle used to measure the presence of combustible gases. The other answer choices are incorrect because: B) an inflatable bag is not used in combustible gas indicators, C) vapor-detecting carbon compounds are not the core technology, and D) sensitive liquid chemicals are not the primary sensing mechanism in these devices.