The correct option is **B. iris**. ### Detailed Explanation: 1. **Understanding the Eye's Structure**: - The human eye is a complex organ that allows us to perceive light and images. It consists of several key parts, including the cornea, iris, pupil, lens, retina, and optic nerve. - The **cornea** is the transparent front part of the eye that helps to focus light. It does not adjust the amount of light entering the eye. - The **retina** is the layer at the back of the eye that contains photoreceptor cells (rods and cones) which convert light into electrical signals sent to the brain. The retina itself does not control light entry. - The **optic nerve** transmits visual information from the retina to the brain. It does not play a role in regulating light entry. 2. **Role of the Iris**: - The **iris** is the colored part of the eye surrounding the pupil. Its primary function is to control the size of the pupil, which in turn regulates the amount of light that enters the eye. - The iris contains muscles that can contract or relax. When there is bright light, the muscles in the iris contract, making the pupil smaller (a process called miosis). This reduces the amount of light that enters the eye, protecting the retina from excessive brightness. - Conversely, in low light conditions, the iris muscles relax, causing the pupil to dilate (a process called mydriasis), allowing more light to enter the eye to improve visibility. 3. **Why Other Options Are Incorrect**: - **A. Cornea**: While the cornea does help focus light, it does not adjust the amount of light entering the eye. Its curvature is fixed and does not change in response to light conditions. - **C. Retina**: The retina is responsible for detecting light and converting it into signals for the brain, but it does not control how much light enters the eye. It is more of a receiver than a regulator. - **D. Optic Nerve**: The optic nerve is crucial for transmitting visual information but does not have any role in controlling light entry. It simply carries the signals from the retina to the brain for processing. ### Summary of Key Points: - The **iris** adjusts the size of the pupil to control light entry into the eye. - The **cornea** focuses light but does not regulate its amount. - The **retina** detects light and sends signals to the brain but does not control light entry. - The **optic nerve** transmits visual information and does not influence light levels. ### Revision Summary: - The iris controls the amount of light entering the eye by adjusting the pupil size. - The cornea focuses light but does not regulate it. - The retina detects light and sends signals to the brain. - The optic nerve transmits visual information but does not control light entry.