Correct Answer: plants

Solution : The correct option is plants .

Oxygen is cycled through the environment in a similar way to how water moves from the sky to the earth and back in the water cycle. Beginning with plants, the oxygen cycle begins. Through a process known as photosynthesis, they transform carbon dioxide and water into carbohydrates and oxygen. To put it another way, plants "breathe" in carbon dioxide and "breathe out" oxygen.