The correct answer is D) gastight bulkheads. Regulations require that cargo pumps in tank vessels carrying grade D liquid cargo be isolated from sources of vapor ignition by gastight bulkheads. This is to prevent the potential ignition of flammable vapors from the cargo by the cargo pumps, which could be a source of ignition. The other answer choices, such as general cargo spaces, cofferdams, or passageways/living quarters, would not provide the same level of isolation and protection against vapor ignition as gastight bulkheads.