**Core Concept**
The Lac operon is a genetic regulatory system in E. coli that controls the expression of genes involved in lactose metabolism. CAP (catabolite activator protein) plays a crucial role in the regulation of the Lac operon by binding to a specific DNA sequence and interacting with RNA polymerase.

**Why the Correct Answer is Right**
CAP is an example of a catabolite activator protein that binds to the lac operator region and helps in the recruitment of RNA polymerase to initiate transcription. When glucose levels are low, CAP is phosphorylated by the enzyme HPr, which enhances its ability to bind to the lac operator and activate transcription. This allows the cell to utilize lactose as an alternative energy source when glucose is scarce.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because CAP is not an example of a repressor protein, which is a type of transcription factor that inhibits gene expression.
* **Option B:** This option is incorrect because CAP is not a transcription factor that binds to the promoter region of the Lac operon; it binds to the operator region.
* **Option C:** This option is incorrect because CAP is not an example of a sigma factor, which is a type of protein that helps in the recognition of specific DNA sequences by RNA polymerase.

**Clinical Pearl / High-Yield Fact**
Understanding the role of CAP in the Lac operon is essential for grasping the fundamental principles of gene regulation in prokaryotes. This knowledge can be applied to the study of various genetic regulatory systems in different organisms.

**Correct Answer: C. CAP is an example of a catabolite activator protein.**