**Core Concept**
Proptosis refers to the forward displacement or protrusion of the globe of the eye from the orbit. It is often caused by an increase in orbital volume, which can result from various pathological processes such as thyroid ophthalmopathy, orbital tumors, or vascular malformations.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of proptosis. Proptosis typically occurs due to an increase in orbital volume, which can be caused by a variety of factors including inflammation, tumors, or vascular malformations. In the case of the correct answer, we are looking for a condition where proptosis is not seen, and this can be due to a lack of increase in orbital volume.

**Why Each Wrong Option is Incorrect**
**Option A:** **Thyroid ophthalmopathy** is a common cause of proptosis, characterized by inflammation and swelling of the orbital tissues, which leads to an increase in orbital volume.
**Option B:** **Orbital cellulitis** is an infection of the orbital tissues, which can cause inflammation and swelling, leading to an increase in orbital volume and proptosis.
**Option C:** **Orbital tumors** such as lymphoma or metastatic disease can also cause proptosis by increasing the volume of the orbit.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that proptosis is often a sign of an underlying orbital pathology, and a thorough evaluation, including imaging studies, is necessary to determine the cause.

**Correct Answer: D. Enophthalmos**