(A) Iris and pupil
The iris controls the size of the pupil, and the pupil regulates the amount of light entering the eye.
The textbook explicitly states: "Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye." Together, iris and pupil are responsible for admitting different amounts of light. Ciliary muscles change focal length, not light admission — a common confusion to avoid.