## Core Concept
The question pertains to the differentiation between stage II and stage III gingivitis, which are classifications within the periodontal disease progression. These stages are characterized by distinct histopathological and clinical features. Understanding the progression and characteristics of gingivitis is crucial for diagnosing and managing periodontal diseases.

## Why the Correct Answer is Right
The correct answer, , involves an understanding that stage III gingivitis is distinguished by a significant increase in the number of plasma cells. Plasma cells are a type of white blood cell that plays a key role in the immune response, particularly in the production of antibodies. The progression from stage II to stage III gingivitis reflects a more intense immune response and inflammation within the gingival tissues. This increase in plasma cells is indicative of a more chronic and severe inflammatory response.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while lymphocytes are present in gingivitis, the key feature that differentiates stage III from stage II gingivitis is more specifically related to the increase in plasma cells rather than lymphocytes in general.
- **Option B:** This option is incorrect because, although neutrophils are a crucial part of the acute inflammatory response in periodontal diseases, the differentiation between stage II and stage III gingivitis is not primarily based on the number of neutrophils.
- **Option C:** This option is incorrect because the question specifically asks about the differentiation feature, which is related to the increase in plasma cells, not macrophages.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the progression of gingivitis to more severe periodontal disease involves an increase in the number of plasma cells within the gingival tissues, indicating a more chronic phase of inflammation. Recognizing this progression is critical for dental professionals to provide appropriate treatment and prevent further disease progression.

## Correct Answer: D. plasma cells