The lac repressor is a protein encoded by the lac I gene. In absence of lactose, the lac repressor binds to the operator sequence of the lac operon and does not allow the RNA polymerase to express the operon. However, as lactose enters the cell from the medium, the beta-galactosidase enzyme converts it into allolactose which in turn serves as an inducer of the lac operon. Binding of allolactose to lac repressor trigger conformational changes in the protein and renders it unable to bind to the operator. The RNA polymerase is allowed to express the operon.
It should be the last one, but if you can pick more than one answer then I would do the first and last one but I’m confident that it would be the last one either way :) hopefully this is helpful
