The focal length of the lens of the eye refers to its ability to focus light onto the retina, allowing us to see objects clearly. In order to adjust the focal length, the eye relies on a mechanism known as accommodation.

Option 1: The iris is responsible for controlling the size of the pupil, which regulates the amount of light entering the eye. While the iris does play a role in controlling the amount of light, it does not directly control the focal length of the lens.

Option 2: The pupil is the opening in the center of the iris that allows light to enter the eye. Similar to option 1, while the pupil affects the amount of light entering the eye, it does not have a direct impact on the focal length of the lens.

Option 3 (Correct Answer): The ciliary body, which is located behind the iris, controls the shape of the lens. By contracting or relaxing the ciliary muscles, the ciliary body changes the shape of the lens and alters the focal length, allowing the eye to focus on objects at different distances. Therefore, option 3 is the correct answer.

Option 4: The cornea is the transparent front part of the eye that refracts light, helping to focus it onto the retina. However, it