Hey there

The correct option is 4) Iris

Explanation for the same is provided below

Aperture is the hole through which light enters the eye. Its size can is changed and controlled by iris. If I tell you about Iris then it is a thin circular structure which controls the diameter and size of the pupil i.e aperture of the eye and regulates the amount of light entering the pupil and reaching the retina as well.

Thankyou