**Core Concept**
Xylitol is a sugar substitute that inhibits the growth of certain bacteria in the oral cavity, specifically those that cause dental caries. This is due to its mechanism as a sugar analog that is not metabolized by these bacteria.

**Why the Correct Answer is Right**
Xylitol's mechanism of action involves its inability to be fermented by oral bacteria, such as Streptococcus mutans, which are major contributors to tooth decay. When these bacteria attempt to metabolize xylitol, they become trapped in the bacterial membrane, leading to cell lysis and ultimately, reduced bacterial growth and incidence of dental caries.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because xylitol does not have a significant effect on the growth of Candida albicans, a fungus that can cause oral infections.
**Option B:** Incorrect because xylitol does not inhibit the growth of Helicobacter pylori, a bacterium that causes stomach ulcers.
**Option C:** Incorrect because xylitol is not known to affect the growth of Escherichia coli, a bacterium commonly found in the gut.

**Clinical Pearl / High-Yield Fact**
Xylitol's use in preventing dental caries is well-established, and it is recommended as a sugar substitute in products aimed at reducing the risk of tooth decay, particularly in children.

**Correct Answer:** C.