Answer: Green plants
Step-by-step explanation:
The constant process of photosynthesis which mopped cabondioixide from the atmosphere (and the activities of phytoplanktons ) and replace this with Oxygen through photolysis of water, and is the main driving force that keep the percentages of atmospheric oxygen relatively constant at 20.95%
Additionally the constant release of Carbon dioxide by animals, and man, as by product of cellular respiration, and the intake of Oxygen ensured constant maintenance of continuous cycle of oxygen for relative constant percentage.