Which structure provides the eye its power of accommodation and has a power of approximately 17 diopters?

The crystalline lens is the primary structure responsible for the eye's accommodation ability, enabling it to focus on objects at varying distances. This lens is flexible and can change its shape, allowing adjustments in focus through contraction and relaxation of the ciliary muscles that surround it.

When focusing on a nearby object, the ciliary muscles contract, causing the lens to become more convex and increase its refractive power, which is approximately 17 diopters when at rest. This increased curvature helps bend light rays more sharply so they can converge on the retina, facilitating clear near vision. Conversely, when focusing on distant objects, the lens flattens, which reduces its power of accommodation.

The other structures listed, such as the lens cortex, cornea, and sclera, do not have the same role in accommodation. The cornea provides most of the eye's total optical power due to its curvature, but it does not change shape to aid in focusing. The lens cortex is a part of the crystalline lens itself and does not independently contribute to accommodation. The sclera is the white outer layer of the eye that provides structure and protection but does not participate in the optical focusing process.