**Core Concept**
The parotid gland is a major salivary gland located in the face, and tumors of the parotid gland are relatively rare. The most common tumor of the parotid gland is benign and arises from the salivary gland tissue.

**Why the Correct Answer is Right**
The most common tumor of the parotid gland is the pleomorphic adenoma, also known as a benign mixed tumor. This tumor arises from the ductal epithelium and the connective tissue of the salivary gland. The pathogenesis is thought to involve the abnormal proliferation of salivary gland cells, leading to the formation of a benign tumor. Pleomorphic adenomas are characterized by their ability to grow slowly and their tendency to recur if not completely excised.

**Why Each Wrong Option is Incorrect**
* **Option A:** Warthin's tumor is a benign tumor of the parotid gland but is less common than pleomorphic adenoma.
* **Option B:** Mucoepidermoid carcinoma is a malignant tumor of the salivary gland, and while it can occur in the parotid gland, it is not the most common tumor.
* **Option C:** Adenoid cystic carcinoma is a malignant tumor of the salivary gland that can occur in the parotid gland, but it is less common than pleomorphic adenoma.

**Clinical Pearl / High-Yield Fact**
Pleomorphic adenomas are the most common tumor of the parotid gland, accounting for approximately 60-70% of all parotid tumors. They can grow slowly and may not cause symptoms for many years, but they can recur if not completely excised.

**Correct Answer: A. Pleomorphic adenoma**