The correct option is
A (a)-(iii), (b)-(iv), (c)-(ii), (d)-(i)
Nitrobacter is a chemoautotrophic bacteria. It converts nitrite into nitrate by oxidation.
Clostridium is a Gram-positive bacteria. They are free living anaerobic nitrogen fixer. They fix atmospheric nitrogen into ammonium compound.
Nitrosococcus is nitrifying bacterium. It converts ammonia into nitrite.
Azotobacter is a free living aerobic nitrogen fixer. It converts atmospheric nitrogen into ammonium compound.
Thus, the correct answer is '(a)-(iii), (b)-(iv), (c)-(ii), (d)-(i).'