**Core Concept**
The question is testing the knowledge of the anatomical dimensions of the human eyeball. The anteroposterior diameter is a critical measurement in ophthalmology, referring to the distance from the anterior (front) surface to the posterior (back) surface of the eyeball.

**Why the Correct Answer is Right**
The correct anteroposterior diameter of a normal adult eyeball is approximately 24 millimeters. This measurement is essential in diagnosing and managing various ocular disorders, such as glaucoma, where changes in the eyeball's dimensions can be indicative of disease progression. The eyeball's shape and size are maintained by the balance of intraocular pressure and the elasticity of the sclera.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely too small, as the anteroposterior diameter is a relatively long dimension of the eyeball.
**Option B:** This option is likely too large, exceeding the normal range for an adult eyeball.
**Option C:** This option is also too large, exceeding the normal range for an adult eyeball.

**Clinical Pearl / High-Yield Fact**
In ophthalmology, the axial length of the eyeball is a critical parameter in calculating intraocular lens power for cataract surgery and in diagnosing conditions such as myopia or hyperopia.

**Correct Answer:** D. 24 millimeters.