Fossil fuel also represent a reservoir of carbon. Carbon cycling occurs through atmosphere, ocean and through living and dead organisms. According to one estimate, 4 × 10¹³ kg of carbon is fixed annually in the biosphere through photosynthesis. A considerable amount of carbon returns to the atmosphere as CO₂ through respiratory activities of the producers and consumers. Decomposers also contribute substantially to CO₂ pool by their processing of waste materials and dead organic matter of land or oceans. Some amount of the fixed carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing CO₂ in the atmosphere.

Human beings return carbon to the atmosphere through:-

Cellular respiration

The correct answer is Option (2) – Cellular respiration

Glucose formation: This process, also known as photosynthesis, occurs in plants and not in humans. It involves the conversion of carbon dioxide (CO₂) and water into glucose and oxygen, and does not return carbon to the atmosphere.

Cellular respiration: This is the correct answer. During cellular respiration, humans and other animals break down glucose and other organic molecules to produce energy. This process releases carbon dioxide (CO₂) as a byproduct, which is then exhaled into the atmosphere.

Nitrogen metabolism: This process involves the transformation of nitrogen-containing compounds in the body. It does not directly return carbon to the atmosphere.

Sweating: Sweating is a mechanism for regulating body temperature and involves the release of water and salts through the skin. It does not involve the release of carbon dioxide or the return of carbon to the atmosphere.

Therefore, cellular respiration is the process through which human beings return carbon to the atmosphere.