This part of the eye dilates and contracts based on the environment

The correct answer is Pupil.

To understand why this is the correct answer, let us look at the function of the pupil and how it interacts with other parts of the eye listed in the options:
1. The Pupil: This is the dark, circular opening in the center of the iris. Its primary function is to regulate the amount of light entering the eye. In bright environments, the surrounding iris muscles contract to constrict (shrink) the pupil, reducing light entry to protect the sensitive retina. In dark or dim environments, the iris muscles relax to dilate (enlarge) the pupil, allowing as much light as possible to enter the eye for better vision.
2. The Sclera: This is the tough, white outer layer of the eyeball. It provides structural support, protection, and maintains the shape of the eye, but it does not contract or dilate.
3. The Cornea: This is the clear, dome-shaped outer surface covering the front of the eye. It acts as a protective window and helps focus incoming light, but its shape is fixed and it does not expand or contract.
4. The Lens: Positioned behind the pupil, the lens changes its shape (thickness) to focus light on the retina (a process called accommodation), but it does not dilate or contract to adjust light intake.

Therefore, the part of the eye that dilates and contracts based on light levels in the environment is the Pupil.