(A) Iris
The iris is a dark muscular diaphragm that controls the size of the pupil, which in turn regulates the amount of light entering the eye.
Source: The Human Eye, Chapter 10, Section 10.1
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Students often confuse iris and pupil here. The iris controls the pupil's size (it is the muscular structure doing the work), while the pupil is merely the opening through which light passes. The question asks what controls the amount of light — that is the iris. The pupil itself regulates light, but only because the iris changes its size. CBSE expects option (A) Iris as the answer based on the textbook wording.