The great oxygen event marks the time, approximately 2.5 billion years ago, when

there was enough oxygen in the atmosphere to support a major burst of life on earth

there was a cataclysmic weather event that absorbed most of the oxygen in the atmosphere and locked it into iron deposits in the oceans

there was significant oxygen in the atmosphere

there was a massive volcanic eruption that lead to a very rapid increase in oxygen levels on earth

photosynthesis was clearly established and producing oxygen.

Answer :There was enough oxygen in the atmosphere to support a major burst of life on Earth

Explanation:

The Great Oxygen Event marks the time, approximately 2.5 billion years ago, when there was enough oxygen in the atmosphere to support a major burst of life on Earth

For the first half of the Earht's history, there was no oxygen in the atmosphere. It was inhabited only by single-celled organisms.Of  those simple life forms, the cyanobacteria may have. evolved a way to take energy from sunlight, and used it to make sugars out of water and carbon dioxide. They used the same chemical process we know as photosynthesis. This released vast quantities of oxygen into the atmosphere and  triggered the evolution of complex life.

i believe it would be c: tons.