Answer:
The correct answer will be option-A.
Explanation:
The lac repressor is a protein produced by the regulator gene present in the lac operon called lacI gene. The repressor proteins bind to a DNA sequence called the operator and block the site for binding of RNA polymerase, thus shuts off the transcription when lactose is absent.
Lac operon is a homotetramer made of two dimers with two alpha-helix bundles which bind to different regions of the operator. The binding of the allolactose to protein causes conformation change in the protein due to which separates the binding helix of the DNA and inhibit the association of the protein to DNA.
Thus, option-A is the correct answer.
