Step 1: Matching products with their microbial sources based on industrial microbiology:
- (a) Vitamin B2 (Riboflavin): Produced by Neurospora gossypii (4).
- (b) Fumaric acid: Produced by Rhizopus arrhizus (1).
- (c) Vitamin B12 (Cyanocobalamin): Produced by Pseudomonas denitrificans (5).
- (d) Lipase: Produced by Candida lipolytica (2).
- (e) Cellulase: Produced by Trichoderma konigii (3).
- (f) Citric acid: Produced by Aspergillus niger (6).
Step 2: Thus, the matches are: (a)-(4), (b)-(1), (c)-(5), (d)-(2), (e)-(3), (f)-(6).