**Core Concept**
Pleomorphic adenoma, also known as a mixed tumor, is a common benign tumor of the salivary glands, particularly the parotid gland. It is the most frequent salivary gland neoplasm, accounting for approximately 60-70% of all cases.

**Why the Correct Answer is Right**
The correct answer is A. Malignant transformation risk is high, because while pleomorphic adenoma is indeed a benign tumor, it has a small but significant risk of malignant transformation. This risk is estimated to be around 1-7% over a period of 10-20 years. The malignant transformation is usually to a carcinoma ex pleomorphic adenoma, which is a type of malignant salivary gland tumor. This risk is a critical consideration in the management of pleomorphic adenoma, and it highlights the importance of complete surgical excision and regular follow-up.

**Why Each Wrong Option is Incorrect**
**Option B:** While pleomorphic adenoma is indeed known as a mixed tumor, this term is more of a descriptive histopathological feature rather than a reflection of its malignant potential. It is characterized by a mixture of epithelial and mesenchymal components.
**Option C:** Pleomorphic adenoma is a benign tumor, but with a small risk of malignant transformation. This risk is a critical consideration in its management.
**Option D:** Pleomorphic adenoma is typically encapsulated, which makes it easier to distinguish from other salivary gland tumors and facilitates complete surgical excision.

**Clinical Pearl / High-Yield Fact**
Pleomorphic adenoma is the most common salivary gland tumor, and it is typically managed with complete surgical excision. However, due to its small risk of malignant transformation, regular follow-up is essential to detect any potential recurrence or malignant transformation.

**✓ Correct Answer: A. Malignant transformation risk is high**