Before entering a fuel tank that has been cleaned, it should be checked with an oxygen indicator and a/an __________.

Correct Answer: Option D — combustible gas indicator

Before entering a cleaned fuel tank, it is mandatory to verify the atmosphere is safe. This requires testing for oxygen content to ensure life support and using a combustible gas indicator to confirm that flammable vapor concentrations are below the LEL, preventing potential explosions or asphyxiation during entry.