Ascientist is studying the gaseous layer of earth. the focus of his/her study is on the

biosphere
hydrosphere
atmosphere
cryosphere

atmosphere

Explanation:

The atmosphere is the gaseous layer of the earth. It consists of mass of gaseous which are collectively called as air. In the air the particles such as dust particles, smoke, soot and chemicals are present. These are attracted by the gravitational pull of the planet. The atmosphere protects the earth from the entry of harmful UV and infrared radiations coming directly from the sun. This way atmosphere protects the living beings from damage caused by these radiations.

On the basis of the above description, atmosphere should be the focus of study.