**Core Concept**
The refractive power of the eye is determined by the shape of the cornea and the lens, which can be altered to change the refractive power. This is achieved through the process of accommodation, where the ciliary muscles relax or contract to change the shape of the lens.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of accommodation, which involves the contraction or relaxation of the ciliary muscles. When the ciliary muscles contract, the zonular fibers relax, allowing the lens to become more spherical and increasing its refractive power. This is essential for near vision. On the other hand, when the ciliary muscles relax, the zonular fibers tighten, causing the lens to become less spherical and decreasing its refractive power, which is necessary for far vision.

**Why Each Wrong Option is Incorrect**
**Option A:** While the lens is involved in changing the refractive power of the eye, it is not the correct answer as the question is asking for a structure that can change its refractive power, and the lens itself does not change its refractive power, but rather the shape of the lens changes.

**Option B:** The retina is the light-sensitive tissue at the back of the eye that converts light into electrical signals, but it does not play a direct role in changing the refractive power of the eye.

**Option C:** The sclera is the white, fibrous outer layer of the eye, but it does not have the ability to change its refractive power.

**Clinical Pearl / High-Yield Fact**
The process of accommodation is essential for near vision, but it can also lead to eye fatigue and strain, especially in individuals who spend extended periods of time focusing on close objects.

**Correct Answer:** C. Ciliary muscles. Ciliary muscles are responsible for changing the shape of the lens to alter the refractive power of the eye.