The power of accommodation is the eye's ability to adjust its focusing power so that objects at different distances all form a sharp image on the retina. Light entering the eye is refracted mainly at the cornea and then by the crystalline lens; for a clear image the combined system must bring the rays to a focus exactly on the retina, whose distance from the lens is fixed. Since that image distance cannot change, the eye must instead vary the lens's own converging power — and it does this by changing the shape (and hence focal length) of the lens using the ciliary muscles. This is precisely what the correct option states.

The mechanism in detail:

• For a nearby object, the ciliary muscles contract, the suspensory ligaments slacken, and the elastic lens bulges to become thicker and more curved. This shortens its focal length and increases its converging power, bending the diverging rays from the near object enough to focus them on the retina.
• For a distant object, the ciliary muscles relax, the ligaments pull the lens thinner and flatter, increasing its focal length and reducing its power — appropriate for the nearly parallel rays arriving from far away.

Why the other statements are incorrect: Saying the cornea provides all the focusing power is wrong because although the cornea supplies most of the eye's fixed refraction, it cannot change; without the lens's adjustment there would be no accommodation at all. The claim that the iris focuses the image confuses two roles — the iris only regulates the size of the pupil and thus the amount of light entering, it does not focus. The idea that the retina moves back and forth is false; the retina's position is fixed, and it is the lens's changing shape, not a moving screen, that accomplishes focusing.