**Core Concept**
The PHP index is a method used for assessing the presence and accumulation of dental plaque on specific tooth surfaces. It is a key tool for evaluating the oral hygiene status of patients and monitoring the effectiveness of oral hygiene practices. The PHP index is a simple and widely used index for scoring plaque.

**Why the Correct Answer is Right**
The PHP index is specifically designed to score plaque on specific tooth surfaces, such as the buccal, lingual, mesial, and distal surfaces of teeth. This index helps in identifying areas where plaque is most likely to accumulate and facilitates targeted oral hygiene interventions. By focusing on plaque scoring, the PHP index provides valuable information for assessing the risk of periodontal disease and guiding preventive measures.

**Why Each Wrong Option is Incorrect**
**Option A:** Evaluating plaque and calculus on specific tooth surfaces is not the primary purpose of the PHP index. While the index does assess plaque, it does not specifically evaluate calculus.
**Option C:** The PHP index does score plaque, but it does not assess gingivitis, which is a separate entity. Gingivitis is an inflammatory condition of the gingiva, often caused by poor oral hygiene.
**Option D:** This option is incorrect because the PHP index does not score calculus or gingivitis. It is specifically designed to evaluate plaque on specific tooth surfaces.

**Clinical Pearl / High-Yield Fact**
The PHP index is an important tool for dental professionals to assess oral hygiene status and guide patients on effective plaque control measures. It is essential to remember that regular monitoring and feedback using the PHP index can significantly improve patient compliance with oral hygiene practices.

**Correct Answer:**
✓ Correct Answer: B. Score plaque on specific tooth surfaces