Nitrification is a process in which ammonia is converted to nitrites and nitrites into nitrates. Examples of nitrifying bacteria includes Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrococcus. Nitrosomonas, Nitrosococcus converts ammonia to nitrites. Nitrobacter, Nitrococcus converts nitrites to nitrates. Denitrification is a process by which nitrate is reduced to molecular nitrogen. Examples of bacteria that carry out denitrification process includes Thiobacillus. Nitrogen fixation is a process by which atmospheric nitrogen is converted into ammonium compounds. Example of free living nitrogen fixer includes Nostoc. Example of free living aerobic nitrogen fixer includes Azotobacter. Thus, the correct answer is 'b and c.'