Which of the following is NOT a common source of thermal energy in combustion?

• A. Chemical
• B. Electrical
• C. Mechanical
• D. Optical

Answer: (D)

Optical energy is not a common source of thermal energy in combustion. In most combustion processes the heat that drives the reaction comes from chemical energy stored in the fuel and released when the fuel reacts with an oxidizer. Electrical energy can provide heat or serve as an ignition source (like a spark or resistance heating), and mechanical energy can be converted to heat through compression or friction to help start or sustain combustion. While optical energy (light) can ignite fuel in rare photochemical scenarios, it is not a typical or reliable heat source for combustion itself.