The correct option is D Co-inducer
Repressors are the transcription factors that bind to operator gene and prevent the RNA polymerase from binding the downstream promoter genes and thus prevent their transcription. A molecule that binds to the repressor and turns off the transcription is called as co-repressor. The inducer is a molecule that binds to the repressor and causes conformational changes in repressor which in turn can no longer bind to the operator. In the presence of lactose, allolactose; an isomer of lactose serves as inducer and 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. Co-inducer turns on the transcription by binding to inducer molecule. The presence of lactose serves to induce binding of inducer to repressor protein which turns the gene expression on, thereby acting as co-inducer. Therefore, the correct answer is option B.