**Core Concept**
The normal axial length of the eye is a critical parameter in ophthalmology, as it affects the power of the lens and the refractive status of the eye. The axial length is defined as the distance from the anterior corneal surface to the retina.

**Why the Correct Answer is Right**
The correct answer, 24mm, is based on the average axial length of a normal human eye. This measurement is obtained using techniques such as ultrasonography or optical coherence tomography (OCT). The axial length is an essential factor in calculating the power of the intraocular lens (IOL) during cataract surgery, as well as in assessing the risk of glaucoma and other ocular disorders. The normal axial length allows for proper accommodation and focus of visual images on the retina.

**Why Each Wrong Option is Incorrect**
**Option A:** 18mm is too short for a normal adult eye, which would lead to hypermetropia (farsightedness) due to the reduced axial length.
**Option B:** 20mm is slightly shorter than the average, but not significantly so, and may not accurately represent the normal axial length in all individuals.
**Option D:** 28mm is significantly longer than the average axial length, which could lead to myopia (nearsightedness) and other ocular complications.

**Clinical Pearl / High-Yield Fact**
A normal axial length of 24mm is essential for proper IOL power calculation and to minimize the risk of postoperative refractive errors.

**✓ Correct Answer: C. 24mm**