Step 1: Understanding the lac operon.
The lac operon is a set of genes involved in the metabolism of lactose in bacteria. In the absence of lactose, the lac operon is repressed by a protein called the lac repressor, which is encoded by the lacI gene. The lac repressor binds to the operator region of the lac operon, preventing transcription of the downstream genes. When lactose is present, it binds to the repressor and inactivates it, allowing transcription to proceed.
Step 2: Conclusion.
Therefore, the repressor of the lac operon is the protein encoded by the lacI gene.