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

The primary function of the pupil in the eye is to control the entry of light. The pupil is the opening in the center of the iris, and it regulates how much light enters the eye based on the surrounding lighting conditions. When it is bright, the pupil constricts to reduce the amount of light entering, which helps protect the retina from excessive brightness and improves visual acuity. In dim lighting, the pupil dilates to allow more light in, ensuring that the retina receives enough illumination for the visual receptors to function effectively.

Other choices refer to functions performed by different structures in the eye. For example, focusing light on the retina is primarily the function of the lens, not the pupil. Nourishing the retina with blood is the role of the retinal blood vessels, and transmitting signals to the brain is done by the optic nerve, receiving visual information from the retina.