The correct answer is option 2: cornea.

The cornea is a thin, transparent membrane located at the front of the eye. It acts as a protective covering for the eye and functions as the main entry point for light into the eye. The cornea plays a vital role in the focusing of light onto the retina, which is essential for clear vision.

The retina, mentioned in option 1, is actually located at the back of the eye and is responsible for converting light into electrical signals that can be processed by the brain to form visual images. Therefore, it is not the membrane through which light enters the eye.

Option 3, the pupil, is an opening in the iris, mentioned in option 4. The pupil adjusts its size to control the amount of light that enters the eye. Although the pupil and iris are important components in regulating the amount of light entering the eye, they are not membranes through which light directly enters.

In conclusion, the cornea is the correct answer as it is the thin membrane through which light enters the eye.