Which of the following is NOT one of the four spheres of the Earth system?

The correct answer is the thermosphere, which is not classified as one of the four main spheres of the Earth system. The four spheres include the atmosphere, hydrosphere, lithosphere, and biosphere.

The atmosphere refers to the layer of gases surrounding Earth, vital for weather and climate systems. The hydrosphere encompasses all of the water on Earth, including oceans, rivers, lakes, and glaciers, playing a crucial role in supporting life and influencing geological processes. The biosphere represents all living organisms and their interactions with the environment, emphasizing the interconnectedness of life and ecosystems.

In contrast, the thermosphere is a layer of the atmosphere that lies above the mesosphere and is characterized by high temperatures and low densities of air. While it is an important part of atmospheric science, it does not represent a separate sphere like the others listed. Hence, recognizing the distinction helps clarify the structure of Earth's systems and their interactions.