The __________ in the back of the eyeball is made up of millions of rods and cones.

The retina is the correct answer because it is the part of the eye located at the back of the eyeball and contains millions of specialized cells known as rods and cones. Rods are responsible for vision in low light conditions and are more sensitive to light, while cones are responsible for color vision and function best in bright light. Together, these photoreceptors convert light into electrical signals that are sent to the brain, allowing us to perceive images.

In contrast, the iris is the colored part of the eye that regulates the size of the pupil, controlling how much light enters the eye. The pupil is the opening in the center of the iris, which also plays a role in light regulation. The lens is a transparent structure that focuses light onto the retina. Each of these parts has a distinct function and is essential for proper vision, but it is the retina that specifically contains the rods and cones responsible for detecting light.