The amount of light that enters the eye is controlled by the muscles of which structure?

The correct choice is the iris, which is a crucial part of the eye that regulates the amount of light that enters. The iris is a muscle that surrounds the pupil, and it can contract or dilate in response to various light conditions. When light levels are high, the iris constricts to make the pupil smaller, thereby reducing the quantity of light that can enter and protecting the sensitive structures of the eye. Conversely, in low light conditions, the iris dilates to enlarge the pupil, allowing more light to enter for better visibility.

While the pupil itself is the opening through which light passes and the cornea and lens help focus light onto the retina, they do not control the amount of light entering the eye. The cornea, although vital for focusing light, remains constant in size, and the lens adjusts its shape to focus images but does not regulate light intensity. Thus, the iris clearly plays the fundamental role of light control in the eye.