**Core Concept**
Open biopsy of salivary gland tumors is generally avoided due to the risk of tumor seeding and tumor spillage, which can compromise the surgical outcome and increase the risk of recurrence. **Fine-needle aspiration cytology (FNAC)** is often used as a diagnostic tool for salivary gland lesions.

**Why the Correct Answer is Right**
The correct answer is related to the type of salivary gland tumor that is most likely to be malignant and has a high risk of perineural invasion. **Pleomorphic adenomas** are the most common type of salivary gland tumor, and they are typically benign. However, they can have a high risk of recurrence if not completely excised.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pleomorphic adenomas are the most common type of salivary gland tumor and are typically benign. They do not require open biopsy.
* **Option B:** Warthin tumors are another type of benign salivary gland tumor and do not require open biopsy.
* **Option C:** Mucoepidermoid carcinoma is a type of malignant salivary gland tumor, but open biopsy is not always necessary for diagnosis.

**Clinical Pearl / High-Yield Fact**
Remember that FNAC is a useful diagnostic tool for salivary gland lesions, but it may not be sufficient for malignant lesions. Core biopsy or open biopsy may be necessary for definitive diagnosis.

**Correct Answer: C. Mucoepidermoid carcinoma.**