**Core Concept**
The question is testing the student's knowledge of orbital masses, specifically the most common type in adults. The retrobulbar orbit is the area behind the eyeball, and masses in this location can cause various symptoms and complications.

**Why the Correct Answer is Right**
The most common retrobulbar orbital mass in adults is a **Varix**, which is a dilated and tortuous vein. This occurs due to the high pressure within the orbital veins, which can cause them to become enlarged and tortuous. The varix can compress the optic nerve and other orbital structures, leading to symptoms such as proptosis, diplopia, and vision loss. The varix is often associated with conditions like Graves' disease, thyroid orbitopathy, and orbital pseudotumor.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cavernous hemangioma is a common orbital mass, but it is more often found in the superotemporal quadrant of the orbit, not typically retrobulbar.
* **Option B:** Lymphoma is a malignant tumor that can occur in the orbit, but it is not the most common retrobulbar orbital mass in adults.
* **Option D:** Carotid-cavernous fistula is a vascular lesion that can cause orbital symptoms, but it is not a retrobulbar orbital mass in the classical sense.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a retrobulbar orbital mass, it is essential to consider the patient's medical history, particularly conditions like Graves' disease and thyroid orbitopathy, as these can increase the risk of developing a varix.

**Correct Answer:** C. Varix