**Core Concept**
The Dean's score is a system used to assess the severity of dental fluorosis. Dental fluorosis is a condition caused by excessive intake of fluoride during tooth development, leading to changes in tooth enamel. The Dean's score is a quantitative measure of the extent and severity of fluorosis.

**Why the Correct Answer is Right**
The Dean's score is calculated based on the percentage of tooth surface affected by fluorosis. The score ranges from 0 (no fluorosis) to 4 (severe fluorosis). A score of 2 corresponds to opaque white areas covering more than 25% but less than 50% of the tooth surface. This score indicates moderate fluorosis. The affected areas may appear as opaque white spots or patches on the tooth enamel. The enamel may also appear dull or chalky.

**Why Each Wrong Option is Incorrect**
* **Option A:** A score of 1 corresponds to very mild fluorosis, where less than 25% of the tooth surface is affected.
* **Option C:** A score of 3 corresponds to severe fluorosis, where more than 50% of the tooth surface is affected.
* **Option D:** A score of 4 corresponds to the most severe form of fluorosis, where the entire tooth surface is affected.

**Clinical Pearl / High-Yield Fact**
It's essential to note that dental fluorosis is a dose-dependent condition, and the severity of the condition can be reduced by limiting fluoride intake during tooth development.

**✓ Correct Answer: B. 2**