**Core Concept**
The Lac operon is a genetic regulatory system in E. coli that controls the expression of genes involved in lactose metabolism. The operon consists of three main structural genes: lacZ, lacY, and lacA, which encode for the enzymes beta-galactosidase, lactose permease, and thiogalactoside transacetylase, respectively. These genes are responsible for the breakdown and utilization of lactose.

**Why the Correct Answer is Right**
The correct answer is A. 3 because the Lac operon contains three structural genes that are responsible for lactose metabolism. The lacZ gene encodes for beta-galactosidase, which breaks down lactose into glucose and galactose. The lacY gene encodes for lactose permease, which transports lactose into the cell. The lacA gene encodes for thiogalactoside transacetylase, which is involved in the metabolism of lactose and its analogs. The absence of these enzymes would prevent the breakdown and utilization of lactose.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because while the Lac operon does contain regulatory genes, such as lacI and lacD, which are involved in the regulation of gene expression, it does not contain four structural genes.

**Option C:** This option is incorrect because the Lac operon does not contain five structural genes. The three main structural genes (lacZ, lacY, and lacA) are sufficient for lactose metabolism.

**Option D:** This option is incorrect because there is no evidence to suggest that the Lac operon contains six structural genes. The three main structural genes (lacZ, lacY, and lacA) are well-established and essential for lactose metabolism.

**Clinical Pearl / High-Yield Fact**
The Lac operon is a classic example of gene regulation in prokaryotes and has been extensively studied in the context of molecular biology and genetics. Understanding the Lac operon is essential for grasping the principles of gene regulation and its applications in biotechnology and medicine.

**✓ Correct Answer: A. 3**