What does '$1$' represent in the given figure?

$A$: Operator gene is functional when it is not blocked by repressor.
$R$: Regulator gene produces active protein only which acts on operon system in $E. coli$.

In $lac$ operon,the switching on or switching off of the operator is achieved by . . . . . . .

Match Column-$I$ with Column-$II$:

Column-$I$	Column-$II$
$(i)$ Operator site	$(a)$ Codes for enzyme (protein)
$(ii)$ Promoter site	$(b)$ Codes for repressor
$(iii)$ Structural gene	$(c)$ Binding site for $RNA$ polymerase
$(iv)$ Regulator gene	$(d)$ Binding site for repressor

At which level is gene expression regulated in the $lac$ operon of $E. coli$?

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