Explanation : (A) Nitrococcus - (II) Conversion of ammonia to nitrite: Nitrococcus is a type of bacteria involved in the nitrification process, specifically, it converts ammonia into nitrite. (B) Rhizobium - (IV) Conversion of atmospheric nitrogen to ammonia: Rhizobium is a type of bacteria found in soil that fix nitrogen after becoming established inside root nodules of legumes. They convert atmospheric nitrogen into ammonia, a process known as nitrogen fixation. (C) Thiobacillus - (I) Denitrification: Thiobacillus is involved in denitrification, the process of reducing nitrates and nitrites to nitrogen gas and returning it to the atmosphere. (D) Nitrobacter - (III) Conversion of nitrite to nitrate: Nitrobacter is a genus of bacteria that play an important role in the nitrogen cycle, where they convert nitrite to nitrate.