**Core Concept**
The question tests the student's knowledge of **vitamin cofactors** in carbohydrate metabolism, specifically in the context of **glycogenolysis**. Glycogen phosphorylase is a key enzyme in this process, breaking down glycogen into glucose-1-phosphate.

**Why the Correct Answer is Right**
The correct answer is **Pyridoxine (Vitamin B6)**, as it acts as a cofactor for glycogen phosphorylase. This enzyme requires pyridoxal phosphate, the active form of vitamin B6, to catalyze the phosphorolytic cleavage of glycogen. The vitamin plays a crucial role in the regulation of glycogen metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine (Vitamin B1) is involved in decarboxylation reactions, not directly in glycogenolysis.
**Option B:** Riboflavin (Vitamin B2) is a component of FAD and FMN, participating in redox reactions, not in glycogen phosphorylase activity.
**Option D:** Niacin (Vitamin B3) is essential for redox reactions and energy metabolism but is not directly related to glycogen phosphorylase.

**Clinical Pearl / High-Yield Fact**
Remember that **pyridoxine deficiency** can lead to impaired glycogenolysis, resulting in **hypoglycemia** and **muscle weakness**.

**Correct Answer:** D. Pyridoxine.