The correct answer is B. Nitrogen.
Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3). This process is essential for life on Earth, as ammonia is a key component of proteins and nucleic acids.
Nitrogen fixation is carried out by a variety of organisms, including bacteria, archaea, and some plants. The most well-known nitrogen-fixing bacteria are the rhizobia, which live in symbiotic relationships with legumes. Rhizobia fix nitrogen in the nodules on legume roots.
Other nitrogen-fixing bacteria include Azotobacter, Clostridium, and Cyanobacteria. These bacteria fix nitrogen in the soil, and they are important in the nitrogen cycle.
Nitrogen fixation is a complex process, and it is not fully understood. However, it is clear that this process is essential for life on Earth.
Oxygen is not fixed by bacteria. It is a byproduct of photosynthesis.
Hydrogen is not fixed by bacteria. It is a byproduct of fermentation.
Carbon dioxide is fixed by bacteria, but this process is called carbon fixation, not nitrogen fixation.