**Core Concept**
The question is testing the student's knowledge of cariogenic potential of various sugars and sweeteners. Cariogenic potential refers to the ability of a substance to promote tooth decay. The underlying principle being tested is the relative ability of different sugars and sweeteners to contribute to the formation of plaque, acid production, and tooth demineralization.

**Why the Correct Answer is Right**
The correct answer is a sugar substitute that is not readily metabolized by oral bacteria, thereby reducing its cariogenic potential. This sugar substitute does not promote the growth of Streptococcus mutans, a key bacterium involved in the formation of dental plaque and acid production. As a result, it does not contribute to tooth demineralization and is considered the least cariogenic option.

**Why Each Wrong Option is Incorrect**
* **Option A:** This sugar is readily metabolized by oral bacteria, leading to the production of acid and tooth demineralization, making it a highly cariogenic substance.
* **Option B:** This sugar substitute is not as effective in reducing cariogenic potential as the correct answer, as it can still be metabolized by oral bacteria to some extent.
* **Option C:** This sugar is highly cariogenic due to its ability to promote the growth of Streptococcus mutans and contribute to acid production and tooth demineralization.

**Clinical Pearl / High-Yield Fact**
When evaluating the cariogenic potential of various sugars and sweeteners, it's essential to consider their relative ability to promote the growth of oral bacteria and contribute to acid production. Sugar substitutes like the correct answer can be a valuable tool in reducing the risk of tooth decay, especially for individuals with a high cariogenic risk.

**Correct Answer: D. Xylitol**