Question

what is the largest natural sink (source or repository) of nitrogen?
the tundra
the desert
the air
the ice caps

Explanation:

Earth's atmosphere (the air) is composed of approximately 78% nitrogen gas ($N_2$), making it by far the largest natural reservoir of nitrogen. Other options like tundra, deserts, and ice caps hold only tiny fractions of the total global nitrogen compared to the atmosphere.

Answer:

C. the air