Trees maintain the balance between gases in the air, removing carbon dioxide from the air and releasing oxygen.

Photosynthesis is correct answer.

Explanation:

Various gases are found in the atmosphere, Oxygen and carbon dioxide are the two most important gases for the survival of plants and animals. Animals and human beings require oxygen to stay alive, while plants need carbon dioxide for producing food and photosynthesis.  Animals breathe the oxygen which is prepared by the plants. Marine animals also take oxygen dissolved in Oxygen. Plants use the process of photosynthesis for producing oxygen, during this process plants take in the carbon dioxide through the tiny openings on their leaves and release the oxygen as a waste product.