The correct answer is: B. Bacteria
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 bacteria, including Rhizobium, Bradyrhizobium, and Azotobacter. These bacteria live in symbiotic relationships with plants, or they can live independently in the soil.
Rhizobium bacteria live in nodules on the roots of legumes, such as beans, peas, and alfalfa. These bacteria convert atmospheric nitrogen into ammonia, which the plant uses to make proteins and nucleic acids. In return, the plant provides the bacteria with a place to live and nutrients.
Bradyrhizobium bacteria live in the soil and fix nitrogen independently. They are often used in agriculture to improve soil fertility.
Azotobacter bacteria also live in the soil and fix nitrogen independently. They are not as common as Rhizobium or Bradyrhizobium bacteria, but they can be used to improve soil fertility.
Viruses, fungi, and protozoa do not fix nitrogen.