Which one of the following is NOT the feature of a primitive...

The primitive atmosphere of the earth was reducing in nature. It primarily consisted of water vapour, methane, ammonia, and hydrogen.

Free oxygen was absent in the primitive atmosphere. Due to the absence of free oxygen, there was no ozone layer to protect the earth from UV radiation.

Nitrogen was mostly present in the form of ammonia, not as free nitrogen gas. The high concentration of free nitrogen (about $78\%$) is a characteristic feature of the present-day atmosphere, not the primitive one.

Answer: Presence of high concentration of Nitrogen.