Match List - I with List - II and choose the correct answer.

List - I	List - 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

To find the correct match, let us analyze the biological functions of each microorganism listed in List - I and match them with the processes in List - II:

(a) Nitrococcus: This is a bacterium involved in the process of nitrification. Specifically, it oxidizes ammonia into nitrite. Thus, (a) matches with (ii) (Conversion of ammonia to nitrite).
(b) Rhizobium: This is a well-known symbiotic bacterium that lives in the root nodules of leguminous plants. It fixes atmospheric nitrogen into ammonia. Thus, (b) matches with (iv) (Conversion of atmospheric nitrogen to ammonia).
(c) Thiobacillus: This bacterium is involved in the process of denitrification, which converts nitrate or nitrite back into gaseous nitrogen compounds (like molecular nitrogen or nitrous oxide), returning it to the atmosphere. Thus, (c) matches with (i) (Denitrification).
(d) Nitrobacter: This bacterium is also involved in nitrification, specifically oxidizing nitrite into nitrate. Thus, (d) matches with (iii) (Conversion of nitrite to nitrate).

Combining these individual matches, we get:
a - (ii), b - (iv), c - (i), d - (iii)