Which part of the eye is not visible?

The retina is the correct answer as it is located at the back of the eye and is not visible from the outside. It is an essential part of the eye responsible for converting light images into electrical signals that the brain can interpret, thus playing a crucial role in vision. The retina contains photoreceptor cells, namely rods and cones, which are vital for detecting light and color.

In contrast, the iris, pupil, and sclera are all visible parts of the eye. The iris is the colored part that controls the size of the pupil, which is the opening that allows light to enter the eye. The sclera is the white outer coating of the eyeball, providing structure and protection. Each of these parts can be seen clearly without any specialized equipment, emphasizing that the retina, being hidden behind these structures, is the only component that isn't visible externally.