The correct answer is B. Cyanobacteria photosynthesizing
Step-by-step explanation:
Oxygen is a chemical element necessary for most life forms to exist, this includes most animals as they require oxygen for multiple body functions. This element is nowadays one of the most abundant elements in the atmosphere and due to this, life forms are abundant and diverse; however, before photosynthetic organisms oxygen was not part of the Earth's atmosphere. This can be explained as organisms that rely on photosynthesis, use carbon dioxide, water, and sunlight and release oxygen into the atmosphere. Due to this, it is believed oxygen in the atmosphere was the result of cyanobacteria and other primitive organisms that carried out photosynthesis.