Endothermic reactions are chemical reactions that absorb heat energy from their surroundings. In other words, they require an input of energy to occur. Among the given options, sweating is the only reaction that is endothermic.

Explanation:

1. Respiration:
Respiration is a metabolic process in which organisms convert glucose and oxygen into carbon dioxide and water, releasing energy in the form of ATP. This process is exothermic, meaning it releases heat energy. It is not an endothermic reaction.

2. Combustion:
Combustion is a reaction between a fuel and an oxidant (usually oxygen), resulting in the release of heat, light, and various products such as carbon dioxide and water. It is an exothermic reaction, as it releases heat energy. Therefore, combustion is not an endothermic reaction.

3. Sweating:
Sweating is a physiological process in which the body releases water through the sweat glands on the skin surface. This process requires energy in the form of heat to convert liquid sweat into vapor, thereby cooling down the body. The absorption of this heat energy from the body's surroundings makes sweating an endothermic reaction. It helps regulate body temperature and prevent overheating.

4. Burning of natural gas:
The burning of natural gas, also known as combustion, is an exothermic reaction. Natural gas (primarily methane) reacts with oxygen to produce carbon dioxide and water, releasing heat energy in the process. Therefore, burning of natural gas is not an endothermic reaction.

In conclusion, among the given options, sweating is the only endothermic reaction. It requires an input of heat energy to occur and helps cool down the body by evaporating sweat. Respiration, combustion, and the burning of natural gas are all exothermic reactions, as they release heat energy.