The focal length of the eye lens increases when eye muscles. When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases.

**Explanation:**

The eye lens is a flexible, transparent structure located behind the iris and pupil of the eye. It plays a crucial role in focusing light onto the retina, allowing us to see objects clearly at different distances. The shape and thickness of the lens can be adjusted by the ciliary muscles, which surround the lens.

1. **Role of the Ciliary Muscles:** The ciliary muscles control the shape and thickness of the lens. These muscles are attached to the lens by tiny ligaments called suspensory ligaments. When the ciliary muscles contract or relax, they cause changes in the shape and thickness of the lens.

2. **Accommodation:** The process by which the lens changes its shape to focus on objects at different distances is known as accommodation. When we focus on a nearby object, the ciliary muscles contract, which releases the tension on the suspensory ligaments. This allows the lens to become thicker and more curved, increasing its refractive power. This adjustment enables the eye to focus the light rays from the nearby object onto the retina, resulting in clear vision.

3. **Relaxation of Eye Muscles:** When the ciliary muscles relax, they pull on the suspensory ligaments, which in turn flatten the lens. This makes the lens thinner and less curved, reducing its refractive power. This adjustment is required for focusing on objects at a distance. The relaxation of the ciliary muscles allows the lens to become flatter, allowing the light rays from the distant object to focus on the retina.

4. **Focal Length and Thinness of the Lens:** The focal length of a lens is the distance between the lens and the point where parallel light rays converge. When the ciliary muscles relax and the lens becomes thinner, the focal length of the lens increases. This means that the lens requires a greater distance to focus light rays onto the retina. This adjustment is necessary to achieve clear vision when looking at objects located at a distance.

Therefore, when the eye muscles are relaxed, the lens becomes thinner, and the focal length increases. This adjustment allows the eye to focus on distant objects. Thus, the correct answer is option **D) Are relaxed and lens becomes thinner**.

When eye muscles (ciliary muscles) contract they stretch eye lens due to which is height increases and breath decreases so it become thinner due to which is principal focus go far and focal length increases