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 nitrate
(c)	Thiobacillus	(iii)	Conversion of nitrite to nitrate
(d)	Nitrobacter	(iv)	Conversion of atmospheric nitrogen to ammonia

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

Let us understand the role of each bacterium involved in the nitrogen cycle step-by-step to match List - I with List - II:

1. Nitrococcus (a):
Nitrococcus is a nitrifying bacterium. It plays a key role in nitrification by oxidizing ammonia into nitrite, which is subsequently converted to nitrate. Therefore, Nitrococcus corresponds to the conversion of ammonia to nitrite/nitrate.
Thus, (a) matches with (ii).

2. Rhizobium (b):
Rhizobium is a well-known symbiotic nitrogen-fixing bacterium. It resides in the root nodules of leguminous plants and converts atmospheric nitrogen into ammonia so that plants can easily assimilate it.
Thus, (b) matches with (iv).

3. Thiobacillus (c):
Thiobacillus (specifically Thiobacillus denitrificans) is a denitrifying bacterium. It reduces nitrates or nitrites present in the soil back into gaseous nitrogen (denitrification), releasing it back into the atmosphere.
Thus, (c) matches with (i).

4. Nitrobacter (d):
Nitrobacter is another nitrifying bacterium. It specifically catalyzes the oxidation of nitrite (${\text{NO}}_2^{-}$) to nitrate (${\text{NO}}_3^{-}$) during the nitrification process.
Thus, (d) matches with (iii).

Combining these findings, we get the correct match:
a - (ii), b - (iv), c - (i), d - (iii)