**Core Concept**
Inulin is a type of fructan, a polysaccharide composed of fructose molecules linked together by beta-2-1 glycosidic bonds. It is a non-digestible carbohydrate found in various plant-based foods, such as chicory root and artichokes, that serves as a prebiotic in the gut.

**Why the Correct Answer is Right**
Inulin is not broken down by human digestive enzymes in the small intestine, but rather reaches the large intestine where it is fermented by colonic bacteria, producing short-chain fatty acids that provide energy to colonic epithelial cells. This process promotes a healthy gut microbiome and supports the growth of beneficial bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Inulin is not a type of fiber, although it is sometimes referred to as a soluble fiber due to its ability to form a gel-like substance in water.
**Option B:** Inulin is not a disaccharide, as it is a polysaccharide composed of multiple fructose molecules.
**Option C:** Inulin is not a sugar substitute, although it is sometimes used as a low-calorie sweetener in food products.

**Clinical Pearl / High-Yield Fact**
Inulin is a rich source of prebiotic fiber that can help promote a healthy gut microbiome and support the growth of beneficial bacteria. It is often used as a dietary supplement to support digestive health and immune function.

**Correct Answer:** D. A non-digestible carbohydrate.