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The choroid is the layer of the eye that is rich in blood vessels, playing a crucial role in providing oxygen and nutrients to the outer layers of the retina. This vascular layer lies between the retina and the sclera and is essential for the metabolic needs of the photoreceptors within the retina. The abundant blood supply from the choroid helps in maintaining the health of the retinal tissues and in removing waste products.

In contrast, the cornea does not contain blood vessels; it is avascular and relies on the diffusion of nutrients from the aqueous humor and the tears for nourishment. The retina, while it plays a significant role in vision, does not have its own blood supply through its layers; instead, it is nourished by the choroidal blood flow and the central retinal artery. The sclera, being the outer protective layer of the eye, is mostly composed of dense connective tissue and has fewer blood vessels, primarily serving structural functions rather than nutritional ones. Therefore, the choroid is the correct answer as it is the primary layer containing blood vessels crucial for the eye's function.