When lactose is present, lactose binds to the repressor, inactivating it so it can no longer bind to the operator.

• A. Operator
• B. Promoter
• C. RNA polymerase
• D. Repressor

Answer: (D)

This question is about how negative regulation controls gene expression in the lac operon. In the absence of lactose, the repressor binds to the operator and blocks transcription. When lactose is present, it is converted to allolactose, which binds to the lac repressor. This binding causes a shape change in the repressor, reducing its affinity for the operator, so the repressor releases from the operator. With the operator free, RNA polymerase can bind at the promoter and transcribe the operon.

So the statement describes the role of the repressor: lactose (via allolactose) inactivates the repressor’s ability to bind the operator, lifting repression. The other components aren’t the direct targets of lactose in this regulatory mechanism—the operator is the site the repressor binds to block transcription, the promoter is where RNA polymerase binds to start transcription, and RNA polymerase is the enzyme that does the transcription once the operator is unblocked.