Final answer:
Transcription remains very low with glucose only, as the presence of glucose allows the repressor to bind to the operator, inhibiting transcription of the lac operon.
Step-by-step explanation:
The combination of lactose and glucose that allows for transcription to be very low due to the binding of the repressor is when there is glucose only (Option b). In this case, the repressor is bound to the operator, which prevents transcription of the lac operon. Even if lactose is present, the bacteria will prefer to use glucose when it is available, thus maintaining the repression of the lac operon. Additionally, the absence of lactose would also result in the repressor binding to the operator, making the option with no lactose or glucose (high cAMP) incorrect. Only when lactose is present and glucose is absent will the lac operon be transcribed. Glucose prevents CAP from binding to the promoter sequence, while lactose prevents the repressor from binding to the operator, allowing transcription to occur.