Nitrification is a process of conversion of ammonium salts to nitrites and nitrites to nitrates. The bacteria that carry out this process are called as nitrifying bacteria. Nitrosomonas, Nitrosospira, Nitrosococcus convert ammonia into nitrites. Nitrobacter, Nitrospina, Nitrococcus convert nitrites into nitrates.
Thus, the correct answer is 'Ammonium salts into nitrates.'