The action of the causes a shift in the focal length of an eye lens.

A pupil is a hole in our eye that allows light to enter.

The iris is a muscle in our eyes that regulates the size of our pupils. It dilates and contracts, causing the pupil size to change.

So we may say that Iris regulates the amount of light that enters our eyes.

The retina is where images are created.

Ciliary muscles are smooth muscle fibres. When the ciliary muscles relax the lens become flat and when they contract the lens grows thicker. The focal length of the eyes changes as the curvature of the eye lens changes.

When the ciliary muscles relax, the eye lens thins and the focus length increases, allowing you to view distant objects.

When the ciliary muscle contracts, the eye lens thickens and the focus length decreases. It allows us to perceive items that are close by.

As a result, option 3 (ciliary muscle) is the right answer.