**Core Concept**
Pulsating exophthalmos is a symptom characterized by the protrusion of the eyeball due to an underlying condition, often resulting in a visible pulsation of the globe. This phenomenon is typically associated with an increase in the volume of the orbital contents.

**Why the Correct Answer is Right**
The most common cause of pulsating exophthalmos is a carotid-cavernous fistula (CCF), which is an abnormal connection between the carotid artery and the cavernous sinus. This fistula leads to an increase in venous pressure within the orbit, resulting in the protrusion of the eyeball. The pulsation is due to the direct transmission of arterial pressure from the carotid artery to the orbital veins.

**Why Each Wrong Option is Incorrect**
**Option A:** Graves' disease is a common cause of exophthalmos, but it typically presents with non-pulsating exophthalmos due to orbital tissue expansion. The exophthalmos in Graves' disease is usually bilateral and not pulsatile.

**Option B:** Orbital cellulitis is an infection of the orbital tissues, which can cause exophthalmos, but it is not typically pulsatile. The exophthalmos in orbital cellulitis is usually associated with signs of infection such as pain, redness, and swelling.

**Option C:** Orbital tumors, such as a meningioma, can cause exophthalmos, but they are not typically pulsatile. The exophthalmos in orbital tumors is usually associated with a gradual increase in orbital volume over time.

**Clinical Pearl / High-Yield Fact**
Pulsating exophthalmos is a classic sign of a carotid-cavernous fistula, which requires prompt evaluation and treatment to prevent complications such as vision loss and stroke.

**Correct Answer: C. Carotid-cavernous fistula.**