In $Escherichia$ $coli$, the product of $i$ gene combines with

The correct option is D Operator to switch off structural genes
$i$ gene
Regulator gene [$lac$ $i$] codes for the repressor protein which binds to the operator region and switches off the operon.
Operator
Operator is a genetic sequence that lies adjacent downstream to the promoter region of DNA and regulates the synthesis of RNA from the downstream structural genes.
The repressor protein binds to the operator region and helps to switch off the operon. This is because the RNA polymerase cannot bind properly to the promoter which has overlaps with the operator region. Hence, the transcription of the downstream structural genes don’t take place.
Final answer
(A) Operator to switch off structural genes