The correct answer is A. Nitrobacter.
Nitrobacter is a chemoautotrophic bacterium that oxidizes nitrite to nitrate. It is a rod-shaped bacterium that is motile by means of flagella. It is found in soil and water, and it is an important part of the nitrogen cycle.
Azotobacter is a genus of bacteria that are able to fix nitrogen. They are aerobic, motile, and Gram-negative rods. They are found in soil and water, and they are an important part of the nitrogen cycle.
Nitosomonas is a genus of bacteria that are able to oxidize ammonia to nitrite. They are aerobic, motile, and Gram-negative rods. They are found in soil and water, and they are an important part of the nitrogen cycle.
Azospirillum is a genus of bacteria that are able to fix nitrogen in association with plants. They are aerobic, motile, and Gram-negative rods. They are found in soil and water, and they are an important part of the nitrogen cycle.
Nitrite is a chemical compound with the formula NO2-. It is a greenish-yellow, water-soluble salt. Nitrite is produced by the oxidation of ammonia by bacteria such as Nitrosomonas. It is also produced by the reaction of nitric acid with metals such as iron. Nitrite is an intermediate in the nitrogen cycle. It is converted to nitrate by bacteria such as Nitrobacter. Nitrite is also used as a food additive. It is used to cure meat and to preserve fish. Nitrite is toxic to humans in high doses. It can cause methemoglobinemia, a condition in which the blood is unable to carry oxygen.