The correct answer is D) combustible gas indicator. Before entering a fuel tank that has been cleaned, it should be checked with an oxygen indicator and a combustible gas indicator. This is to ensure that the tank is free of hazardous vapors and has sufficient oxygen levels to support safe entry. A combustible gas indicator is used to detect the presence of flammable or explosive gases, which is a crucial safety precaution when working in confined spaces like fuel tanks. The other options are incorrect because a halide torch (A) is used for leak detection, a flame safety lamp (B) is not suitable for detecting combustible gases, and an Orsat apparatus (C) is used for analyzing the composition of gases, not for detecting the presence of combustible gases.