**Core Concept**
The adult crystalline lens is a flexible, transparent structure located behind the iris and the pupil, playing a crucial role in refracting light onto the retina for clear vision. The lens's thickness is essential for its refractive power.

**Why the Correct Answer is Right**
The adult crystalline lens is approximately 10 millimeters thick. This thickness allows for the optimal refractive power necessary for clear vision. The lens's elasticity and flexibility enable it to change shape and adjust its refractive power in response to changing light conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as 5 millimeters is too thin for the adult crystalline lens, which would result in inadequate refractive power.
**Option B:** Incorrect, as 15 millimeters is excessively thick, which would lead to an overemphasis on refractive power and potentially cause vision problems.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the adult crystalline lens's refractive power is influenced by its shape, with the lens becoming more spherical with age, contributing to age-related presbyopia.

**Correct Answer: C. 10 millimeters**