Final answer:
Oxygen began to accumulate in Earth's atmosphere approximately 2 billion years ago, largely due to photosynthesis from cyanobacteria, leading to the formation of the protective ozone layer.
Step-by-step explanation:
Oxygen began to accumulate in Earth's atmosphere about 2 billion years ago, which is answer option B. This significant increase in free oxygen led to the formation of the ozone layer, composed of a triple molecule of oxygen known as O3. The creation of the ozone layer was essential as it provides protection against the Sun's harmful ultraviolet light. Prior to the oxygen accumulation, primarily initiated by photosynthesis from cyanobacteria and other phototrophic organisms, the atmosphere lacked free oxygen, and life was confined to the oceans. The transition to an oxygen-rich atmosphere enabled life forms to venture onto land and significantly altered the course of evolution on our planet.
Free oxygen began accumulating in the atmosphere about 2 billion years ago, and the increased amount of this gas led to the formation of Earth's ozone layer (recall that ozone is a triple molecule of oxygen, 03), which protects the surface from deadly solar ultraviolet light. Before that, it was unthinkable for life to venture outside the protective oceans, so the landmasses of Earth were barren.