**Core Concept:**
The Lac Operon is a group of genes in bacteria that are activated under low glucose conditions. The primary regulator proteins involved in the Lac Operon are the catabolite activator protein (CAP) and the repressor protein (R). CAP is a transcription factor that binds to the promoter region of the Lac Operon genes and enhances their transcription when glucose levels are low.

**Why the Correct Answer is Right:**
The correct answer, **Option D**, is the repressor protein (R). The repressor protein binds to the operator region of the Lac Operon genes and prevents the binding of CAP to the promoter region, thus preventing transcription of the Lac Operon genes. When glucose levels are high, the repressor protein binds to the operator region, preventing CAP from binding and keeping the Lac Operon genes inactive.

**Why Each Wrong Option is Incorrect:**
Option A ("DNA polymerase") is incorrect because DNA polymerase is a crucial enzyme involved in DNA replication and repair, not related to the Lac Operon or glucose regulation.

Option B ("RNA polymerase") is incorrect because RNA polymerase is the enzyme that synthesizes RNA from DNA templates, not a regulatory protein for glucose control.

Option C ("Lactose permease") is incorrect because lactose permease is a transporter protein that facilitates the uptake of lactose into the cell, but it is not a regulatory protein involved in glucose control.

**Clinical Pearl:**
The Lac Operon and its regulatory proteins play a crucial role in the adaptive response of bacteria to changing glucose concentrations. By controlling the expression of genes involved in lactose utilization, bacteria can efficiently utilize lactose as a carbon source when available, and switch to glucose utilization when glucose levels rise. This allows bacteria to conserve energy and adapt to fluctuating glucose concentrations in their environment.