The correct answer is option 4 - Phytoplanktons. Phytoplanktons are microscopic organisms that live in oceans, lakes, and rivers. They are responsible for producing approximately 50-85% of the Earth`s oxygen through the process of photosynthesis. During this process, phytoplanktons use sunlight, carbon dioxide, and water to produce glucose and oxygen. This oxygen is released into the environment and is vital for supporting life on Earth.

Options 1, 2, and 3 - Deserts, green forests, and grasslands, while important ecosystems, do not produce as much oxygen as phytoplanktons. Deserts have minimal plant life, green forests do produce oxygen through photosynthesis but not at the same scale as phytoplanktons, and grasslands have a lower density of plants compared to forests. Therefore, phytoplanktons are the primary source of maximum oxygen on Earth.