**Core Concept**
Thyroid Eye Disease (TED) is a complex, autoimmune inflammatory condition affecting the extraocular muscles and surrounding tissues, often associated with Graves' disease. The ocular manifestations of TED can significantly impact vision and quality of life.

**Why the Correct Answer is Right**
The most common ocular manifestation of TED is **Exophthalmos**, also known as proptosis, which is the forward displacement of the globe due to increased volume of the orbital contents. This occurs as a result of inflammation and swelling of the orbital tissues, including the extraocular muscles, fat, and connective tissue. The increased pressure and volume within the orbit cause the eyeball to protrude from the orbit, leading to exophthalmos. This is often accompanied by eyelid retraction, lacrimation, and photophobia.

**Why Each Wrong Option is Incorrect**
**Option A:** Enophthalmos is the sinking of the eyeball into the orbit, often seen in cases of orbital trauma or fracture, and is not a common manifestation of TED.
**Option B:** Strabismus refers to misalignment of the eyeballs, which can be a complication of TED due to muscle inflammation and weakness, but it is not the most common ocular manifestation.
**Option C:** Diplopia, or double vision, is a common symptom of TED, but it is not the most common ocular manifestation. It occurs due to muscle imbalance and weakness.

**Clinical Pearl / High-Yield Fact**
In TED, early recognition and treatment of exophthalmos can prevent further complications, such as optic nerve compression, which can lead to permanent vision loss.

**Correct Answer: C. Strabismus.**