Which layer of Earths atmosphere merges with the vacuum 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.

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