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The primary function of the choroid layer of the eye is to provide nutrients to the retina. The choroid is a vascular layer located between the retina and the sclera (the white outer layer of the eyeball) and is rich in blood vessels. Its main role is to supply oxygen and nutrients to the outer layers of the retina, which is crucial for maintaining the health and function of this light-sensitive tissue. The choroid also helps to absorb excess light, preventing reflections within the eye that could interfere with vision.

While the absorption of light is important in the overall function of the eye, it is not the primary function of the choroid itself. The other choices involve functions such as protection and regulation of the pupil size, which are handled by other structures in the eye, like the sclera and the iris, respectively. Therefore, focusing on nutrient supply accurately highlights the essential role of the choroid in ocular health and visual processing.