The correct answer is B) They must be explosion-proof, self-contained, battery-fed lamps. This is correct because the regulations for the use of portable electric lights in petroleum product tanks require that they be explosion-proof and self-contained, using a battery power source. This is to prevent the risk of ignition of flammable vapors that may be present in the tank. The other options are incorrect because A) is too broad, as explosion-proof lamps can be used even when the compartment is not gas-free; C) is too restrictive, as explosion-proof lamps are allowed; and D) is incorrect, as the fixture must be explosion-proof but a ground wire is not required.