The correct answer is A. Carbon dioxide gas (CO2) is best for extinguishing fires in engine compartments. The reasoning is that CO2 gas is an effective fire extinguishing agent for Class B fires, which include flammable liquid and gas fires. Engine compartment fires often involve fuel or oil, making them Class B fires. CO2 gas effectively smothers the fire by displacing oxygen, without leaving any residue that could damage engine components. The other options are not as suitable. Water (B) and dry chemical (C) extinguishers can cause damage to electrical and mechanical components in the engine compartment. Foam (D) is effective on Class B fires but may not be appropriate in tight engine spaces.