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 match the microorganisms in List - I with the organic acids they produce in List - II, we can analyze each pairing:

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

(b) Acetobacter aceti: This is a bacterium known for converting ethanol into acetic acid, commonly known as vinegar. Therefore, (b) matches with (i).

(c) Clostridium butylicum: This is a bacterium used for the production of butyric acid. Therefore, (c) matches with (iv).

(d) Lactobacillus: This bacterium converts lactose and other sugars into lactic acid, which is essential in curd formation. Therefore, (d) matches with (ii).

Matching these together, we get:
a - (iii)
b - (i)
c - (iv)
d - (ii)

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