Exothermic reactions release heat energy (ΔH < 0)

Steps:

• Define exothermic as a reaction that releases energy, typically as heat, with negative enthalpy change.
• Identify combustion reactions as generally exothermic due to bond breaking and forming.
• Evaluate option A: methane combustion (CH₄ + 2O₂ → CO₂ + 2H₂O) releases heat.
• Compare with others: C and D require energy input, making them endothermic.

Why A is correct:

• Combustion of methane follows the general combustion enthalpy law, where ΔH = -890 kJ/mol, confirming heat release.

Why the others are wrong:

• B: While combustion of hydrocarbons is exothermic, the question specifies methane as the precise example; B is overly broad.
• C: Electrolysis decomposition absorbs electrical energy, so ΔH > 0 (endothermic).
• D: Photosynthesis uses light energy to form glucose, requiring energy input (endothermic).

Final answer: A