What is the primary function of the choroid in the eye?

The primary function of the choroid in the eye is to supply blood to the outer layers of the retina. The choroid is a vascular layer located between the sclera (the white part of the eye) and the retina. It contains a rich network of blood vessels that are essential for providing nutrients and removing waste products from the retinal layers. This blood supply is crucial for the metabolic needs of the retina, particularly for the outer layers, which are responsible for absorbing light and transmitting visual signals. The health and function of the choroid are vital for maintaining proper vision, as any compromise could lead to retinal degeneration or other visual impairments.

The other functions mentioned in the other choices pertain to different structures in the eye. For instance, focusing light onto the retina is primarily the role of the lens and cornea, while the regulation of pupil size is managed by the iris. Storing vitreous humor is a function associated with the vitreous body, which is the gel-like substance filling the space between the lens and the retina. Thus, option B accurately identifies the choroid's role within the overall structure and function of the eye.