Which part of the eye appears as a circular opening in the center?

The part of the eye that appears as a circular opening in the center is the pupil. The pupil is the opening that allows light to enter the eye, enabling vision. It is surrounded by the iris, which is the colored part of the eye. The size of the pupil can change depending on the amount of light in the environment, expanding or contracting to help regulate how much light reaches the retina at the back of the eye.

The cornea is the clear, dome-shaped front surface of the eye that helps focus light, but it does not appear as a circular opening. The iris, while it does have a circular shape, is the muscle that controls the size of the pupil rather than being an opening itself. The lens, located behind the iris, further focuses light but is not visible as an opening either. Thus, the pupil is the correct answer due to its defining role as the actual opening through which light passes into the eye.