Question

what is the atmosphere?
all of the water on earth
all of the land on earth
all of the air on earth
the part of earth where all life exists

Explanation:

The atmosphere is a layer of gases surrounding the Earth. It is composed of air. "All of the water on Earth" refers to the hydrosphere. "All of the land on Earth" is the geosphere. "The part of Earth where all life exists" is the biosphere.

Answer:

All of the air on Earth