Nitrosomonas changes :
A. Nitrite to nitrate
B. Nitrogen to ammonia
C. Ammonia to nitrogen
D. Ammonia to nitrite

Hint: Nitrosomonas is a nitrifying bacteria. Nitrosomonas play a crucial function in giving nitrogen to plants and restricting the fixation of carbon dioxide. They are widely allocated in water or soil, where there are huge quantities of ammonia, such as in streams or lakes into which untreated and treated sewage is pumped.

Complete answer:
Nitrifying bacteria are a tiny organization of aerobic bacteria that use the inorganic chemical as a source of energy. These bacteria are valuable in the nitrogen cycle for the conversion of ammonia into nitrate. This process occurs in two groups:
First, bacteria that is responsible for the conversion of ammonia into nitrite are named as Nitrosomonas, Nitrosospira and Nitrosococcus, the process is known as nitrification and bacteria that is responsible for the oxidation of nitrite into nitrate are named as Nitrobacter, Nitrospina and Nitrococcus.
A bacteria that is responsible for the conversion of nitrogen into ammonia is named as Rhizobium. These bacteria serve as nitrogen-fixing bacteria. When nitrogen has served their purpose in plants, specialized decomposing bacteria will initiate a process known as ammonification ( ammonia back into ammonium salt).
After nutrients are converted back into ammonia, anaerobic bacteria will convert ammonia back into the nitrogen gas and the process is known as Denitrification.

Hence, the correct answer is option (D).

Note: Nitrite is more toxic than nitrate. Even Nitrite is used as a source of food by plants. Nitrifying bacteria comprises two types of bacteria: Ammonia oxidising bacteria and nitrite oxidising bacteria. They are chemolithotrophic organisms. By the oxidation of inorganic nitrogen compounds, these bacteria will obtain their energy.