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

List - I	List - II
(a) Aspergillus niger	(i) Acetic acid
(b) Acetobacter aceti	(ii) Lactic acid
(c) Clostridium butylicum	(iii) Citric acid
(d) Lactobacillus	(iv) Butyric acid

To solve this matching problem, we need to associate each microorganism in List - I with the organic acid it produces in List - II. Let's analyze them one by one:

(a) Aspergillus niger: This is a filamentous fungus widely used in biotechnology for the commercial production of citric acid. Therefore, (a) matches with (iii).

(b) Acetobacter aceti: As the name suggests, this bacterium oxidizes ethanol into acetic acid, which is the main component of vinegar. Therefore, (b) matches with (i).

(c) Clostridium butylicum: This anaerobic bacterium is well-known for fermenting carbohydrates to produce butyric acid. Therefore, (c) matches with (iv).

(d) Lactobacillus: This genus of lactic acid bacteria ferments sugars into lactic acid, which is crucial in food industries, particularly for making curd and yogurt. Therefore, (d) matches with (ii).

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

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