Which cycle involves processes that make nitrogen usable for living organisms?

The nitrogen cycle is the correct choice because it encompasses the series of processes that convert nitrogen in the atmosphere into forms that can be utilized by living organisms, particularly plants. Nitrogen in the atmosphere is abundant but exists primarily in a gaseous form (N2), which most organisms cannot directly use. The nitrogen cycle includes several key processes such as nitrogen fixation, where nitrogen gas is converted into ammonia or related compounds, and nitrification, where ammonia is oxidized to nitrates that plants can absorb.

Furthermore, the cycle also involves other processes like assimilation, where plants take up these nitrates, and denitrification, which returns nitrogen to the atmosphere, maintaining a balance. This cycle is essential for all forms of life as nitrogen is a critical component of amino acids, proteins, and nucleic acids.

While the carbon cycle, hydrogen cycle, and oxygen cycle involve other essential elements for life and their transformations in the ecosystem, they do not specifically address the processes that make nitrogen usable for organisms. The focus of the nitrogen cycle on nitrogen's conversion and availability emphasizes its unique role in supporting life, distinct from the other cycles mentioned.