In eukaryotic organisms,gene expression regulation cannot occur at which of the following levels?

The $Lac-Operon$ model was elucidated by

Which of the following enzymes increases the permeability of the bacterial cell to lactose?

Which of the following statements is incorrect regarding the $lac$ operon?

In a negative operon,

An organism is able to survive on a culture medium containing nutrient $A$ through an enzyme-catalyzed process: $A \xrightarrow{X} B \xrightarrow{Y} C$. $A$ mutant organism fails to survive on this medium. However,it grows when nutrient $B$ is added to the medium. Which gene of this mutant is defective?