**Core Concept**
Dietary fibers are non-digestible carbohydrates that provide various health benefits, including promoting regular bowel movements, satiety, and blood sugar control. They are classified into two main categories: soluble and insoluble fibers.

**Why the Correct Answer is Right**
Riboflavin, also known as vitamin B2, is a water-soluble vitamin that plays a crucial role in energy production and can be found in various food sources, including dairy products and leafy green vegetables. It is not a dietary fiber, but rather a micronutrient essential for human health. Unlike pectin, cellulose, and hemicellulose, which are all types of dietary fibers, riboflavin is not a component of plant cell walls or a component that provides bulk to the diet.

**Why Each Wrong Option is Incorrect**
**Option A:** Pectin is a type of soluble dietary fiber found in the cell walls of fruits, particularly in the skin and core of apples. It is known for its gel-like properties and is often used as a gelling agent in food products.
**Option B:** Cellulose is the most abundant dietary fiber in plant cell walls, providing structure and rigidity to plant cells. It is an insoluble fiber that helps promote regular bowel movements and prevent constipation.
**Option C:** Hemicellulose is a type of dietary fiber that is found in the cell walls of plants, particularly in the bark and wood of trees. It is an insoluble fiber that helps provide bulk to the diet and promote regular bowel movements.

**Clinical Pearl / High-Yield Fact**
Dietary fibers can be broadly classified into two categories: soluble and insoluble fibers. Soluble fibers, such as pectin and beta-glucans, can help lower cholesterol levels and regulate blood sugar levels, while insoluble fibers, such as cellulose and hemicellulose, can help promote regular bowel movements and prevent constipation.

**Correct Answer:**
✓ Correct Answer: D. Riboflavin