In the $lac$ operon, what does the term "$lac$" refer to?

Which of the following acts as an inducer for the $lac$ operon in $Escherichia$ $coli$?

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

In the medium where $E$. coli was growing, lactose was added, which induced the lac operon. Then, why does the lac operon shut down some time after the addition of lactose in the medium?

$Y$ gene in $lac$ operon is responsible for .....

Which of the following statements is correct regarding the repressor of the $lac$-operon?