**Core Concept**
Pleomorphic adenoma, also known as a benign mixed tumor, is a common salivary gland tumor that can recur due to its unique histopathological characteristics. The tumor's growth pattern and capsule formation play a crucial role in its recurrence.

**Why the Correct Answer is Right**
The correct answer is "Incomplete capsule" because pleomorphic adenoma often has an incomplete or fragmented capsule, which is a key factor in its recurrence. When the capsule is incomplete, tumor cells can spill out and implant on adjacent tissues, leading to recurrence. This is a result of the tumor's growth pattern, where it can grow in a pushing or infiltrative manner, making complete excision challenging.

**Why Each Wrong Option is Incorrect**
**Option A:** Metastasis is not a common mechanism of recurrence for pleomorphic adenoma, as it is a benign tumor. Metastasis is typically associated with malignant tumors.
**Option C:** Pleomorphic adenoma is not considered a highly aggressive tumor, and its recurrence is not due to its aggressive nature. While it can recur, it does not typically exhibit aggressive behavior such as rapid growth or invasion.

**Clinical Pearl / High-Yield Fact**
Incomplete capsule formation is a hallmark of pleomorphic adenoma, making complete excision challenging. Surgeons should strive to remove the tumor with a margin of normal tissue to minimize the risk of recurrence.

**Correct Answer:**
✓ Correct Answer: B. Incomplete capsule