Match List-I with List-II and choose the correct answer from the options given below.

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

Correct Answer: The correct option matching List-I with List-II is a - iii, b - i, c - iv, d - ii.

Explanation and Logical Breakdown:

To solve this matching question, let us analyze each microorganism listed in List - I along with its corresponding commercial acid product in List - II:

1. (a) Aspergillus niger
Aspergillus niger is a fungus used commercially for the large-scale production of Citric acid.
⇒ Matches with (iii) Citric acid.

2. (b) Acetobacter aceti
Acetobacter aceti is a bacterium that oxidizes ethanol into acetic acid, used in the production of Acetic acid (vinegar).
⇒ Matches with (i) Acetic acid.

3. (c) Clostridium butylicum
Clostridium butylicum is an anaerobic bacterium capable of producing Butyric acid through fermentation.
⇒ Matches with (iv) Butyric acid.

4. (d) Lactobacillus
Lactobacillus species (commonly known as lactic acid bacteria or LAB) ferment lactose into Lactic acid.
⇒ Matches with (ii) Lactic acid.

Final Combination:
(a) → (iii)
(b) → (i)
(c) → (iv)
(d) → (ii)

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