**Core Concept**
A cavernous hemangioma is a benign orbital tumor characterized by its well-defined capsule, slow growth, and retrobulbar location within the muscle cone. These tumors are composed of vascular spaces that are typically filled with blood and are the most common primary orbital tumor in adults.

**Why the Correct Answer is Right**
Cavernous hemangiomas are the most common primary orbital tumors in adults, typically presenting in the 2nd to 4th decade of life. They are well-circumscribed and encapsulated, allowing for easy resection. The slow growth of these tumors leads to a gradual and progressive proptosis (bulging of the eye). The tumor's location within the muscle cone, which is the area behind the rectus muscles of the eye, is a characteristic feature of cavernous hemangiomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemangiopericytoma is a rare vascular tumor that can occur in the orbit but is not as common as cavernous hemangioma and often lacks a well-defined capsule.

**Option B:** Dermoid cysts are a type of benign orbital tumor but are typically located in the superotemporal quadrant of the orbit and are not easily resectable due to their complex anatomy.

**Option C:** Capillary hemangiomas are more common in infants and young children and are typically characterized by a rapid growth and a more diffuse, poorly defined capsule.

**Clinical Pearl / High-Yield Fact**
Cavernous hemangiomas are the most common primary orbital tumor in adults, and their characteristic presentation of slow-growing proptosis should prompt a high index of suspicion for this diagnosis.

**Correct Answer:**
✓ Correct Answer: D. Cavernous hemangioma