What was the major external cause for loss of light gases from the early atmosphere?

Correct answer: Solar wind

Solar wind was the major external factor that helped remove light gases from the early atmosphere. It consists of a continuous stream of charged particles flowing outward from the Sun. In the young Solar System, the early Earth had a weaker atmosphere and a less effective protective magnetic environment, so light gases could be swept away more easily into space. River deposition and organic manure are Earth-surface processes, while human industry developed billions of years later and cannot explain the loss of the primitive atmosphere. Therefore, solar wind is the only scientifically appropriate answer, making option C correct.