Questions: oxygen

The answer is the last one: oxygen.

Explanation for each option:

• Methane: Methane is believed to have been present in the early Earth's atmosphere. It is a simple organic molecule that could have been produced by volcanic activity and other geological processes.

• Water vapor: Water vapor was likely present on early Earth, as it is a fundamental component of the planet's hydrosphere and would have been released into the atmosphere through volcanic outgassing.

• Nitrogen: Nitrogen is a major component of Earth's atmosphere today and was likely present in significant amounts on early Earth as well. It is a relatively inert gas that would have been stable in the early atmosphere.

• Oxygen: Oxygen was likely not present in significant amounts on early Earth. The early atmosphere is thought to have been reducing, with little free oxygen. Oxygen levels increased significantly only after the Great Oxidation Event, which occurred around 2.4 billion years ago, due to the photosynthetic activity of early life forms like cyanobacteria.

In summary, oxygen was likely not present on early Earth in significant amounts.