Which type of reaction releases heat?

• A. Endothermic reaction
• B. Isothermal reaction
• C. Adiabatic reaction
• D. Exothermic reaction

Answer: (D)

Heat released in a reaction occurs when the products end up at a lower energy than the reactants, so the extra energy is given off to the surroundings as heat. This is what characterizes an exothermic reaction: the enthalpy change is negative, meaning the system loses energy and the surroundings typically get warmer. In contrast, endothermic reactions absorb heat from the surroundings (positive enthalpy change), while isothermal and adiabatic descriptions refer to how heat exchange with the surroundings is managed—constant temperature in isothermal, and no heat transfer in adiabatic—rather than whether heat is released during the reaction. So the best answer is the exothermic reaction, which inherently releases heat.