The correct answer is C) expand in volume. When fuel oil is heated, it will expand in volume. This is due to the basic principle of thermal expansion, where materials increase in volume as they are heated. As the fuel oil molecules gain kinetic energy from the heat, they move further apart, causing the overall volume of the fuel oil to increase. The other answer choices are incorrect because: A) Specific gravity would decrease, not increase, as the volume expands. B) Viscosity would decrease, not increase, as the fuel oil becomes less dense. D) Specific heat refers to the amount of energy required to raise the temperature of a substance, not the change in volume.