**Core Concept:** The lens of the eye is a transparent, spherical structure located behind the iris and in front of the vitreous humor. Its primary function is to focus light onto the retina, enabling clear vision. The lens composition includes water, proteins, and solutes like Ca2+ and Mg2+.

**Why the Correct Answer is Right:** The correct answer, **D**: 81% is the water content in the lens. The lens is composed mainly of water, which maintains its transparency and flexibility. The high water content allows for easy changes in lens shape, essential for focusing light and maintaining accommodation (the ability to change the lens' curvature to focus on nearby or distant objects).

**Why Each Wrong Option is Incorrect:**

A. **83%**: This is too high, making the lens too rigid and less able to change shape, affecting accommodation and vision.
B. **72%**: This is too low, reducing transparency and making the lens less effective in focusing light.
C. **90%**: Similar to option A, this would make the lens too rigid, limiting accommodation and vision.

**Clinical Pearl:** Maintaining the proper water content in the lens is crucial for maintaining vision and accommodating to different distances. Dehydration or excess water intake can lead to changes in lens water content, potentially affecting vision.

**Correct Answer:** 81%