What part of the Earth is mostly composed of nitrogen?

The atmosphere is the correct choice because it is the layer of gas surrounding the Earth, and nitrogen is the most abundant gas found in this layer, making up approximately 78% of the atmosphere's composition. This composition is crucial for various processes, including supporting life and regulating climate.

In contrast, the lithosphere refers to the Earth's solid outer layer, which is primarily composed of rock and minerals. The hydrosphere encompasses all the water found on Earth, including oceans, rivers, and glaciers, where nitrogen is present but not in significant amounts. The biosphere involves all living organisms and their interactions with the environment, which includes the use of nitrogen but is not itself primarily composed of nitrogen. Understanding these distinctions helps clarify why the atmosphere is identified as the part of the Earth rich in nitrogen.