The correct answer is D) expand in volume. When fuel oil is heated, its molecules move farther apart, causing the fuel to expand in volume. This is a characteristic property of liquids - they tend to expand when heated due to the increased kinetic energy of their molecules. The other answer choices do not accurately describe the behavior of fuel oil when heated. A) is incorrect because specific heat is a measure of the amount of heat required to raise the temperature of a substance, not its volumetric expansion. B) is incorrect because specific gravity, which is the ratio of a substance's density to the density of water, would decrease as the fuel expands in volume. C) is incorrect because viscosity, which is a measure of a fluid's resistance to flow, would actually decrease as the fuel oil is heated and becomes less viscous.