According to Beadle and Tatum's hypothesis, 4 genes are necessary for this pathway.
Option 4 is correct.
Beadle and Tatum's one gene-one enzyme hypothesis states that each gene codes for an enzyme that is involved in a specific step of a metabolic pathway. This means that the number of genes in a pathway is directly related to the number of steps in the pathway.
Mutations in the genes encoding these enzymes can lead to defects in the pathway. A mutation in the gene encoding the enzyme that converts A to B would prevent the conversion of A to B, and the entire pathway would be blocked.
Beadle and Tatum's hypothesis is very influential in our understanding of genetics and biochemistry and has helped to explain how cells can carry out complex biochemical reactions, and it has been used to identify and characterize many genes involved in metabolic pathways.