The correct answer is option 'C' - Nitrogen cycle.

The Nitrogen cycle is the process by which the amount of nitrogen remains the same in the atmosphere. It is an essential component of the Earth's ecosystem and is crucial for the survival of all living organisms. Nitrogen is a key element for the formation of proteins, nucleic acids, and other essential compounds in living organisms.

The nitrogen cycle involves several steps and processes that occur in different parts of the environment, including the atmosphere, soil, water bodies, and living organisms. These steps include nitrogen fixation, nitrification, assimilation, ammonification, and denitrification.

- Nitrogen Fixation:
This is the process by which atmospheric nitrogen (N2) is converted into a usable form by certain bacteria called nitrogen-fixing bacteria. These bacteria convert nitrogen gas into ammonia (NH3) through a process called nitrogen fixation. Some examples of nitrogen-fixing bacteria include Rhizobium, Azotobacter, and Cyanobacteria. These bacteria live in the soil or in symbiotic relationships with plants.

- Nitrification:
Nitrification is the process by which ammonia (NH3) is converted into nitrites (NO2-) and then into nitrates (NO3-). This process is carried out by nitrifying bacteria. Nitrites and nitrates are soluble forms of nitrogen that can be easily taken up by plants for growth.

- Assimilation:
Assimilation is the process by which plants and animals take up nitrogen from the environment and incorporate it into their own tissues. Plants absorb nitrates from the soil through their roots and use them to synthesize proteins and other nitrogen-containing compounds. Animals obtain nitrogen by consuming plants or other animals.

- Ammonification:
Ammonification is the process by which organic nitrogen compounds, such as proteins and nucleic acids, are broken down and converted into ammonia (NH3) by decomposer bacteria and fungi. This ammonia can then be further converted into nitrites and nitrates through nitrification.

- Denitrification:
Denitrification is the process by which nitrates (NO3-) are converted back into nitrogen gas (N2) and released into the atmosphere. This process is carried out by denitrifying bacteria that live in oxygen-poor environments, such as waterlogged soil or sediments.

These various processes in the nitrogen cycle ensure that the amount of nitrogen in the atmosphere remains constant. Nitrogen is continuously cycled between the atmosphere, plants, animals, soil, and water bodies, providing a constant supply of this essential element for all living organisms.