**Core Concept**
Proptosis, or exophthalmos, refers to the forward displacement or bulging of the globe of the eye out of the orbit. This condition can result from various causes, including inflammatory, infectious, or neoplastic processes. The underlying anatomical structure responsible for proptosis is the orbital contents, which include the eyeball, eyelids, lacrimal gland, and extraocular muscles.

**Why the Correct Answer is Right**
Proptosis occurs when there is an increase in the volume of the orbital contents or a decrease in the volume of the orbit. This can be due to several factors, including thyroid ophthalmopathy, which is characterized by inflammation and swelling of the tissues around the eye, including the eyelids, lacrimal gland, and extraocular muscles. The increased volume of these tissues pushes the eyeball forward, resulting in proptosis. The orbital fat and extraocular muscles play a crucial role in this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Carotid-cavernous fistula is a condition that can cause proptosis, but it is not the most common cause. Carotid-cavernous fistula is a rare vascular abnormality that can cause increased intraorbital pressure and proptosis.
**Option B:** Orbital cellulitis is an infection of the orbital tissues that can cause proptosis, but it is not the most common cause. Orbital cellulitis can result from the spread of infection from the sinuses or other adjacent structures.
**Option C:** Orbital tumors, such as neuroblastoma or rhabdomyosarcoma, can cause proptosis, but they are not the most common cause. Orbital tumors can arise from the orbital tissues or can metastasize to the orbit.

**Clinical Pearl / High-Yield Fact**
Thyroid ophthalmopathy is a common cause of proptosis, especially in patients with Graves' disease. This condition can result in significant morbidity, including diplopia, vision loss, and cosmetic disfigurement.

**Correct Answer:** C. Orbital tumors.