**Core Concept**
Birbeck granules are distinctive organelles found in the cytoplasm of Langerhans cells, a type of dendritic cell. These granules are composed of a rod-shaped core surrounded by a membrane and are crucial for the processing and presentation of antigens to T-cells.

**Why the Correct Answer is Right**
Birbeck granules are formed through the fusion of vesicles containing antigens and lysosomes, which leads to the formation of a rod-shaped core. This structure is essential for the presentation of antigens to T-cells through major histocompatibility complex (MHC) class II molecules. The presence of Birbeck granules is a key diagnostic feature of Langerhans cell histiocytosis, a disease characterized by the abnormal proliferation of Langerhans cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because Birbeck granules are not characteristic of melanocytes, which contain melanosomes instead.
* **Option B:** Incorrect because Birbeck granules are not associated with keratinocytes, which contain keratin filaments.
* **Option C:** Incorrect because Birbeck granules are not found in plasma cells, which contain secretory granules.

**Clinical Pearl / High-Yield Fact**
Birbeck granules are a valuable diagnostic tool in the histopathological examination of skin biopsies, particularly in the diagnosis of Langerhans cell histiocytosis. Their presence can help differentiate Langerhans cell histiocytosis from other conditions that may present with similar clinical features.

**Correct Answer: C. Langerhans cells**