**Core Concept**
Xylitol is a sugar substitute that is used as a sweetener in various products, including sugar-free gum, candy, and baked goods. It is a type of sugar alcohol that is not metabolized by bacteria in the mouth, which makes it beneficial for oral health.

**Why the Correct Answer is Right**
Xylitol works by inhibiting the growth of Streptococcus mutans, a type of bacteria that is responsible for tooth decay. When xylitol is consumed, it is not metabolized by the bacteria, and instead, it is not used as a source of energy. This leads to an increase in the pH of the mouth, making it less acidic and less conducive to the growth of these bacteria. Xylitol also stimulates the production of saliva, which helps to wash away bacteria and food particles from the teeth.

**Why Each Wrong Option is Incorrect**
**Option A:** Xylitol is a carbohydrate. This is incorrect because xylitol is a sugar alcohol, which is a type of carbohydrate but has distinct properties that make it useful as a sweetener.

**Option B:** Xylitol is a sugar. This is incorrect because xylitol is not a simple sugar like sucrose or glucose, but rather a sugar alcohol that is derived from the hydrogenation of xylose.

**Option C:** Xylitol is a preservative. This is incorrect because xylitol is used primarily as a sweetener, not as a preservative.

**Clinical Pearl / High-Yield Fact**
Xylitol is not toxic to humans, but it can be toxic to dogs, who can develop liver failure if they ingest large amounts of xylitol-containing products.

**Correct Answer:** C. A sugar alcohol.