Answer: The ciliary muscle
Step-by-step explanation:
The ciliary muscle is a ring-shaped muscle located around the circumference of the eye's lens. It is responsible for changing the shape of the lens by contracting or relaxing.
When the ciliary muscle contracts, it loosens the zonules (fibers that hold the lens in place) that attach the lens to the ciliary body. This allows the lens to become flatter, which is necessary for focusing on distant objects. On the other hand, when the ciliary muscle relaxes, the zonules tighten, causing the lens to become thicker and more rounded. This allows the eye to focus on close objects.
This process is called "accommodation", by which the eye is able to adjust the shape of the lens to focus on objects at different distances.