**Core Concept**
The gingiva is a specialized mucous membrane covering the alveolar part of the mandible and maxilla, playing a crucial role in maintaining oral health. The gingiva is composed of various cell types, including epithelial cells, fibroblasts, and immune cells, which work together to provide a protective barrier against bacterial invasion.

**Why the Correct Answer is Right**
The major cells seen in the gingiva are epithelial cells, fibroblasts, and immune cells, such as macrophages and lymphocytes. Epithelial cells form the surface layer of the gingiva, providing a physical barrier against bacteria. Fibroblasts are responsible for producing collagen and other extracellular matrix components, which give the gingiva its strength and elasticity. Immune cells, such as macrophages and lymphocytes, help to eliminate pathogens and maintain immune homeostasis in the gingiva.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the type of cells present in the gingiva.
**Option B:** This option is incorrect as it is not a cell type, but rather a tissue component.
**Option C:** This option is incorrect as it is a type of connective tissue found in other parts of the body, but not the primary cell type in the gingiva.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the gingiva is a dynamic tissue that undergoes continuous remodeling in response to changes in the oral environment. Understanding the cell types present in the gingiva is crucial for developing effective treatments for periodontal diseases.

**Correct Answer: D. Epithelial cells, fibroblasts, and immune cells.**