Final answer:
A strain mutant for the gene encoding enzyme b would be able to grow on medium supplemented with enzyme b only.
Step-by-step explanation:
If a, b, and c are all required for growth, and you have a strain mutant for the gene encoding enzyme b, then this strain would be unable to produce enzyme b on its own and therefore would require supplementation with enzyme b for growth. This is in line with the one gene-one enzyme hypothesis proposed by Beadle and Tatum, which suggests that each gene is responsible for producing a specific enzyme. When a gene is mutated and can't produce its enzyme, the pathway the enzyme is involved in gets interrupted. To restore growth in the case of a pathway required for survival, the missing enzyme must be provided. Therefore, the mutant strain would be able to grow on medium supplemented with enzyme b only.