**Core Concept**
The length of the eyeball is a critical anatomical measurement that determines the overall dimensions of the eye and its relationship with surrounding structures. This measurement is essential for understanding the normal anatomy and potential pathologies of the eye. The length of the eyeball is typically measured from the corneal surface to the posterior aspect of the eyeball.

**Why the Correct Answer is Right**
The average length of the adult human eyeball is approximately **24 millimeters**. This measurement is crucial for assessing the normal anatomy of the eye and for diagnosing conditions such as microphthalmia (small eye) or hyperopia (farsightedness). The length of the eyeball is also related to the axial length, which is the distance from the anterior surface of the cornea to the posterior pole of the eye. A longer axial length can contribute to conditions such as myopia (nearsightedness).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not provide a specific measurement. While it may be a range, it does not give a precise value.
* **Option B:** This option is incorrect as it is not a commonly accepted measurement for the length of the eyeball. It may be a measurement for a different part of the eye or a related structure.
* **Option C:** This option is incorrect as it is not a typical measurement for the length of the eyeball. It may be a measurement for a different part of the eye or a related structure.

**Clinical Pearl / High-Yield Fact**
Remember that the axial length of the eye is a critical factor in determining the refractive error of the eye. A longer axial length is associated with myopia, while a shorter axial length is associated with hyperopia.

**Correct Answer:** C. 24 millimeters.