The correct option is C Lactose
The lac z, lac y and lac genes in lac operon are the structural genes that code for beta-galactosidase (converts lactose into glucose and galactose), beta-galactoside permease (transports lactose into the cell) and beta-galactoside transacetylase respectively. In the presence of lactose, allolactose; an isomer of lactose, binds to the repressor and causes an allosteric shift. The altered repressor cannot bind to the operator, allowing RNA polymerase to transcribe the lac genes. Thus lactose is required for expression of the lac operon. The correct answer is C.