Which was least likely to have been a component of Earth's atmosphere before life began? A-sulfurB-carbon dioxideC-oxygenD-water vapor

The correct answer is Oxygen.

The surface of the Earth was liquefied. As the Earth cooled, an atmosphere generated primarily from gases erupted from volcanoes. It constituted methane, hydrogen sulfide, and ten to 200 times as much as carbon dioxide in comparison to today's atmosphere.

It was considered that there was an extreme volcanic activity for the initial billion years of the existence of the Earth. The initial atmosphere was possibly majority carbon dioxide will no or little concentration of oxygen. There were slight quantities of ammonia, water vapor, and methane.