Regulator gene regulates the expression of structural genes by its protein products that are mostly transcription factors. These transcription factors bind to the operator region which is located just upstream the binding site of the RNA polymerase and hence prevent the binding and transcription. The lac operon has one regulatory gene which is present upstream the operator gene. The genes that code for any RNA / protein product which do not serve as regulatory protein are known as structural genes. The lac z, lac y and lac a genes in lac operon are the structural genes. Option B is the correct answer.