Correct Option: B. Lens Detailed Explanation: The lens of the eye is a transparent, flexible structure located behind the iris and the pupil. Its primary function is to focus light onto the retina, which is the light-sensitive layer at the back of the eye. Here’s how the lens works and why it is the correct answer:

1. Light Refraction: When light enters the eye, it first passes through the cornea, which provides most of the eye's optical power due to its curvature. However, the cornea does not have the ability to change its shape significantly. The lens, on the other hand, can change its shape to adjust the focus of light rays. This process is known as accommodation.
2. Accommodation: The lens is controlled by the ciliary muscles. When these muscles contract, the lens becomes thicker (more convex), allowing it to focus on nearby objects. Conversely, when the ciliary muscles relax, the lens flattens, which is necessary for focusing on distant objects. This ability to change shape is crucial for clear vision at varying distances.
3. Focusing Light on the Retina: Once the light is properly focused by the lens, it travels to the retina, where photoreceptor cells (rods and cones) convert the light into electrical signals. These signals are then sent to the brain via the optic nerve, allowing us to perceive images.

Why Other Options Are Incorrect:

• A. Cornea: While the cornea does play a significant role in focusing light, it primarily provides the initial refraction of light entering the eye. It does not have the ability to change shape like the lens does, which is essential for focusing on objects at different distances. Therefore, while the cornea is important, it is not the main structure responsible for fine-tuning focus.
• C. Retina: The retina is not involved in focusing light; rather, it is the surface that receives the focused light. The retina contains photoreceptors that detect light and convert it into neural signals. It is crucial for vision but does not play a role in the focusing process itself.
• D. Iris: The iris is the colored part of the eye that controls the size of the pupil, thereby regulating the amount of light that enters the eye. While it is important for controlling light exposure, it does not focus light onto the retina. Its function is more about light regulation than focusing.

Summary of Key Points:

• The lens is responsible for focusing light onto the retina by changing its shape (accommodation).
• The cornea provides initial refraction but does not adjust focus.
• The retina receives focused light and converts it into signals for the brain.
• The iris regulates light entry but does not contribute to focusing.

This understanding of the structures of the eye and their functions is essential for grasping how vision works and is a fundamental concept in biology related to human anatomy and physiology.