Answer: The lac operon is an inducible operon that is activated in the presence of lactose.
An operon is a unit of genes which are controlled by the single promoter and operator sequence. It consists of a regulatory gene, promoter, operator, and structural genes.
Lac (lactose) operon consists of three structural genes namely lacZ, lacY, and lacA. These genes are transcribed and translated to produce specific enzymes used for lactose metabolism.
It is called as inducible operon because lactose acts like an inducer as its presence induces the transcription process. The lactose is first converted into allolactose which then binds to repressor (synthesized by regulatory gene). In this way, it makes the repressor inactive and therefore allows the transcription of structural genes.