**Core Concept**
Snyder's Test is a method used to assess the cariogenic potential of saliva. The test involves applying saliva samples to a series of cups containing a substrate, and observing the color change over time. This color change is indicative of the substrate's breakdown by the enzymes present in the saliva.

**Why the Correct Answer is Right**
The correct answer is 24 hours because Snyder's Test is designed to detect the presence of high levels of salivary amylase, an enzyme that breaks down starches into sugars. These sugars can then be fermented by oral bacteria, leading to the production of acid and subsequent color change. The presence of salivary amylase is indicative of a highly cariogenic saliva, which can contribute to the development of dental caries.

**Why Each Wrong Option is Incorrect**
**Option B:** 48 hours is incorrect because, while some color change may be observed at this time point, it is not indicative of a highly cariogenic saliva. The enzyme activity and substrate breakdown may be slower at this time point, leading to a less pronounced color change.
**Option C:** 72 hours is incorrect because, by this time, the substrate may have broken down completely, leading to a false negative result. This option is often used as a distractor to test the student's understanding of the time frame required for Snyder's Test.
**Option D:** 96 hours is incorrect because it is an excessively long time frame for Snyder's Test. The test is typically conducted over a shorter period, and observing color change at this time point would be unusual.

**Clinical Pearl / High-Yield Fact**
Snyder's Test is a useful tool for assessing the cariogenic potential of saliva, but it is not a definitive diagnostic test. The results should be interpreted in conjunction with other clinical findings and risk factors for dental caries.

**✓ Correct Answer: A. 24**