Match Column - I with Column - II.

	Column-I		Column-II
(a)	Nitrococcus	(i)	Denitrification
(b)	Rhizobium	(ii)	Conversion of ammonia to nitrite
(c)	Thiobacillus	(iii)	Conversion of nitrite to nitrate
(d)	Nitrobacter	(iv)	Conversion of atmospheric nitrogen to ammonia

Choose the correct answer from options given below.

The correct option is (a) - (ii) , (b) - (iv), (c) - (i), (d) - (iii).

To understand this matching, let us analyze the roles of each microorganism involved in the nitrogen cycle step-by-step:
(a) Nitrococcus: This bacterium is a chemoautotrophic nitrifying bacterium. It plays a key role in nitrification by oxidizing ammonia (NH₃) into nitrite (NO₂^-). Thus, (a) matches with (ii).
(b) Rhizobium: This is a well-known symbiotic bacterium found in the root nodules of leguminous plants. It is responsible for biological nitrogen fixation, which is the conversion of inert atmospheric nitrogen (N₂) into ammonia (NH₃) so that plants can readily absorb it. Thus, (b) matches with (iv).
(c) Thiobacillus: This bacterium participates in denitrification. Denitrification is the process of converting nitrates (NO₃^-) back into gaseous atmospheric nitrogen (N₂), completing the nitrogen cycle. Thus, (c) matches with (i).
(d) Nitrobacter: This is another nitrifying bacterium that acts in the second step of nitrification. It oxidizes nitrite (NO₂^-) into nitrate (NO₃^-). Thus, (d) matches with (iii).