**Core Concept**
The submandibular gland is one of the salivary glands, producing both serous and mucous secretions. Tumors of the submandibular gland can be benign or malignant, and understanding their characteristics is essential for diagnosis and management.

**Why the Correct Answer is Right**
The most common tumor of the submandibular gland is a benign condition known as **pleomorphic adenoma**, also referred to as a benign mixed tumor. This tumor arises from the gland's ductal epithelium and is characterized by its ability to differentiate into various cell types, including epithelial and mesenchymal components. The pathogenesis of pleomorphic adenoma involves the activation of various signaling pathways, including the Wnt/β-catenin pathway, which promotes cell proliferation and differentiation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Warthin's tumor is another benign tumor of the salivary gland, but it is more commonly associated with the parotid gland.
* **Option B:** Mucoepidermoid carcinoma is a malignant tumor of the salivary gland, but it is less common than pleomorphic adenoma in the submandibular gland.
* **Option D:** Adenoid cystic carcinoma is a malignant tumor of the salivary gland, but it is not the most common tumor of the submandibular gland.

**Clinical Pearl / High-Yield Fact**
Pleomorphic adenoma is the most common salivary gland tumor, accounting for approximately 60-70% of all salivary gland tumors. It is essential to differentiate pleomorphic adenoma from other salivary gland tumors, as it has a relatively benign course and can be treated with surgical excision.

**Correct Answer: C. Pleomorphic adenoma**