Match the columns.

(a)	Nitrobacter	(i)	Free living aerobic nitrogen fixer
(b)	Clostridium	(ii)	Converts ammonia into nitrite
(c)	Nitrosococcus	(iii)	Changes nitrite to nitrate
(d)	Azotobacter	(iv)	Free living anaerobic nitrogen fixer

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).'