Option 1 -> Respiration is exothermic as it releases energy when glucose breaks down with oxygen.
Option 2 -> Burning of natural gas is a combustion reaction that releases heat and light energy, making it exothermic.
Option 3 -> Decomposition of vegetable matter is exothermic as microorganisms release heat during the breakdown process.
Option 4 -> Thermal decomposition requires continuous heat input to break chemical bonds and decompose compounds into simpler substances.
Hence, Option 4: Thermal decomposition -> is an endothermic reaction because it needs external heat energy to break down substances (e.g., CaCO₃ → CaO + CO₂ requires heating). All other options release heat energy, making them exothermic reactions -> correct