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

How many operons are present in the $nif$ gene cluster of $Klebsiella$ $pneumoniae$?

How is the $lac$ operon regulated by the repressor?

In eukaryotes,the regulation of gene expression could be exerted at $:-$
$(I)$ Transcriptional level (formation of primary transcript)
$(II)$ Processing level (regulation of splicing)
$(III)$ Transport of $\text{mRNA}$ from nucleus to the cytoplasm
$(IV)$ Translational level
Choose the correct order for given statements $:-$

$A$: $Lac$ operon exerts negative control only.
$R$: The operator is occupied by aporepressor during regulation.

Which of the following is the substrate for $\beta$-galactosidase?