Which layer of earths atmosphere merges with the vacuum of space?

The exosphere, being the outmost layer of the atmosphere, gradually merges with the vaccum of space.

Exosphere

Explanation:

The exosphere refers to the extreme outermost layer of earth. It is at an altitude of about 500-10,000 km above the earth's surface. This layer is comprised of lighter elements such as helium, hydrogen and also a small amount of carbon is present. It is so high up in the sky that the upper region of this layer shares a boundary with the space. Here, the air is very limited, drastically very thin and are scattered randomly. This layer receives a large amount of incoming solar radiation, as a result of which the temperature of this layer is very high along with the layer of thermosphere.

Thus, the correct answer is Exosphere.