The correct answer is d: transcription can be turned off when a repressor binds to the DNA near the promoter.
The repressor is a protein which prevents a gene from being expressed (inhibits transcription from occurring). Near the promoter, there is a site called the operator, which is the binding site for the repressor. When the repressor binds, RNA polymerase is prevented from binding to the promoter and initiating transcription. Gene expression is then turned off.
