What best describes biologists' current hypothesis about the production of the Earth's atmospheric oxygen? 

A)  Photosynthesis by terrestrial plants produces atmospheric oxygen. 
B)  The breakdown of iron ore deposits produces atmospheric oxygen. 
C)  Photosynthesis by photosynthesizing bacteria produces atmospheric oxygen. 
D)  Chemosynthesis by aquatic plants produces atmospheric oxygen.

Atmospheric Oxygen

The oxygen gas (O2) present in the Earth's atmosphere, essential for the respiratory processes of most living organisms.

Photosynthesizing Bacteria

Microorganisms that convert light energy into chemical energy through the process of photosynthesis.

Terrestrial Plants

Plants that grow on land, as opposed to aquatic (water-based) or epiphytic (growing on other plants) plants.