Step 1: Understanding the Concept:
The Lac Operon is a classic model of gene regulation in bacteria, controlling the metabolism of lactose.
Step 2: Detailed Explanation:
An inducer is a molecule that initiates gene expression by interacting with a repressor protein.
In the absence of lactose, the Lac repressor binds to the operator, preventing transcription of the structural genes ($lacZ$, $lacY$, $lacA$).
When lactose enters the cell, it is converted into an isomer called allolactose.
Allolactose (or lactose in broader terms) acts as the inducer by binding to the repressor.
This binding causes the repressor to change shape and release the DNA, allowing RNA polymerase to begin transcription.
While allolactose is the molecular inducer, "Lactose" is typically used as the answer in general microbiology contexts.
Step 3: Final Answer:
In the Lac Operon system, Lactose acts as the inducer molecule.