The correct answer is D) mechanical energy. In an internal combustion engine, the thermal energy produced during the combustion of fuel is transformed into mechanical energy, which is then used to power the engine and move the vehicle. The chemical energy stored in the fuel is converted into heat energy during the combustion process, and this heat energy is then used to drive the engine's pistons, crankshaft, and other moving parts, ultimately producing the mechanical energy that propels the vehicle forward. The other answer choices are incorrect because they do not accurately describe the energy transformation that occurs in an internal combustion engine. Combustion energy (A) is the initial energy released during the fuel combustion process, while internal energy (B) and external energy (C) are not the primary forms of energy produced by the engine.