Which one of the following acts as an inducer for lac operon ?

Lactose acts as an inducer for the lack operon. The lac operon is a system found in bacteria, such as Escherichia coli, that regulates the expression of genes involved in lactose metabolism. The operon consists of regulatory elements, including a promoter, an operator and three structural genes: lacZ, lacY and lacA.
(1), (3), (4) Sucrose, glucose and galactose are not directly involved in the induction of the lac operon. Lactose is broken down by the enzyme $\beta \beta \beta$-galactosidase to produce glucose and galactose.