Which part of the eye is responsible for controlling the size of the pupil?

The iris is the part of the eye that controls the size of the pupil. It is a thin, circular structure made of muscle fibers that can contract or relax. When the iris muscles contract, they make the pupil smaller, which helps to limit the amount of light that enters the eye in bright conditions. Conversely, when the iris relaxes, the pupil becomes larger, allowing more light to enter in darker conditions. This regulation is crucial for protecting the retina from excessive light and for adjusting vision clarity under varying light conditions.

The cornea primarily functions to focus light onto the retina and does not have a role in adjusting pupil size. The lens also focuses light but works in conjunction with the cornea rather than controlling the pupil. The retina is the light-sensitive layer at the back of the eye that transmits visual information to the brain, but it does not regulate pupil size either. Hence, the iris is the correct answer as it is specifically designed for this function.