When this is present in the cell, it binds to the repressor and the repressor can no longer bind to the operator:

• A. Operon.
• B. Inducer.
• C. Promoter.
• D. Repressor.
• E. Corepressor.

Answer: (B)

Verified step by step guidance

1. Understand the concept of an operon: An operon is a cluster of genes under the control of a single promoter and is regulated together. It includes structural genes, a promoter, an operator, and regulatory genes.
2. Identify the role of a repressor: A repressor is a protein that binds to the operator region of an operon to prevent transcription of the genes. When bound, it blocks RNA polymerase from transcribing the genes.
3. Learn about the inducer: An inducer is a molecule that binds to the repressor, causing a conformational change that prevents the repressor from binding to the operator. This allows transcription to proceed.
4. Differentiate between corepressor and inducer: A corepressor binds to the repressor to enhance its ability to bind to the operator, thus inhibiting transcription. In contrast, an inducer binds to the repressor to prevent it from binding to the operator, thus allowing transcription.
5. Apply this knowledge to the problem: The question asks for the molecule that binds to the repressor and prevents it from binding to the operator. Based on the definitions, the correct answer is 'Inducer'.