**Core Concept**
The most common cause of proptosis in adults is a type of orbital disease that involves the expansion of a tissue or mass behind the eye, leading to its forward displacement. This condition is often associated with the growth or infiltration of abnormal cells within the orbit.

**Why the Correct Answer is Right**
The most common cause of proptosis in adults is thyroid ophthalmopathy, also known as Graves' orbitopathy. This condition is an autoimmune disease that affects the thyroid gland, leading to the production of autoantibodies that stimulate the orbital tissue behind the eye. The increased production of glycosaminoglycans and collagen in the orbital tissue results in its expansion, causing the eye to protrude from the orbit. The orbital tissue involved in this process includes the extraocular muscles, lacrimal gland, and orbital fat.

**Why Each Wrong Option is Incorrect**
**Option A:** Orbital cellulitis is a bacterial infection of the orbital tissue, which can cause proptosis, but it is not the most common cause in adults.
**Option B:** Orbital lymphoma is a type of cancer that can cause proptosis, but it is relatively rare compared to thyroid ophthalmopathy.
**Option C:** Orbital metastasis refers to the spread of cancer from another part of the body to the orbit, which can cause proptosis, but it is not the most common cause in adults.

**Clinical Pearl / High-Yield Fact**
Thyroid ophthalmopathy can be associated with other autoimmune conditions, such as hyperthyroidism, Hashimoto's thyroiditis, and pernicious anemia. A thorough medical history and physical examination are essential in diagnosing the underlying cause of proptosis.

**Correct Answer:** D. Thyroid ophthalmopathy.