In the carbon cycle, the human body returns carbon to the atmosphere through this way:

The correct option is cellular respiration.

In the carbon cycle, carbon is constantly exchanged between the atmosphere, oceans, soil, and living organisms. The human body, like other animal bodies, participates in this cycle by consuming carbon-rich organic compounds (like carbohydrates, proteins, and fats) and releasing carbon back into the atmosphere.

The primary biological process through which humans return carbon to the atmosphere is cellular respiration. During cellular respiration, cells break down glucose in the presence of oxygen to produce usable energy. The chemical equation for this process is represented as follows:

$$C_6{H}_{12}{O}_6+6O_2\to 6{\mathrm{CO}}_2+6H_{2}O+\text{Energy (ATP)}$$

As shown in the equation, carbon dioxide (CO₂) is produced as a metabolic byproduct. This carbon dioxide is transported through the bloodstream to the lungs and exhaled, returning carbon directly back to the atmosphere in a gaseous form.

Let's look at why the other options are incorrect:
• Formation of glucose: This is an anabolic process where carbon is stored in organic molecules, not released into the atmosphere.
• Photosynthesis: This process is performed by plants, algae, and some bacteria, which removes carbon dioxide from the atmosphere rather than returning it, and cannot be performed by the human body.
• Waste products: While human waste contains carbon, it enters the decomposer pathway in soil or water systems rather than being the direct physiological mechanism by which the active human body returns gaseous carbon to the atmosphere.