The correct answer is option 3: Ciliary muscles.

The ciliary muscles are responsible for changing the shape of the lens in the eye, which allows for the eye to focus on objects at different distances. When an object is far away, the ciliary muscles relax, causing the lens to become flatter, allowing the eye to focus on distant objects. This is called distant vision.

On the other hand, when an object is close, the ciliary muscles contract, which causes the lens to become more curved, enabling the eye to focus on nearby objects. This is called near vision.

The iris, mentioned in option 1, is the colored part of the eye and is responsible for controlling the size of the pupil. The iris does not play a direct role in near and far vision.

Option 2 suggests that the convex nature of the lens is responsible for near and far vision. While the lens`s shape does contribute to focusing light onto the retina, it is the action of the ciliary muscles that allows the lens to change its shape for near and far vision.

Option 4 suggests that the retina is adjustable. However, the retina is the layer of tissue at the back of the eye that receives and processes visual information, and it is not the main factor