Final answer:
A mutant strain defective in the gene encoding enzyme A requires supplementation with nutrient B for growth, as it can't synthesize B from A but can convert B to C and other required products.
Step-by-step explanation:
If A, B, and C are all required for growth in a simple metabolic pathway, and a strain is mutant for the gene encoding enzyme A, meaning it cannot produce A on its own, then in order for this strain to grow, the medium must provide what the mutant strain cannot synthesize.
Given that enzymes usually work in a sequential order within a metabolic pathway, supplying the mutant with nutrient A alone will not suffice since the enzyme to convert A to B is defective.
Therefore, the strain would need to be supplied with nutrient B, so it can convert B to C and continue with the growth process.
By providing nutrient B, the pathway's block is effectively bypassed, allowing the organism to grow.
The other supplements are unnecessary since the strain is only deficient in converting A to B, not B to C or C to a further product.