(b) Pupil
The pupil is the part of the human eye that allows light to enter into the eye by regulating and controlling the amount of light entering it.
Source: Chapter 10, Section 10.1 – The Human Eye
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Although light first touches the cornea (which is technically the entry point and where most refraction occurs), the question asks specifically about the part that allows/regulates light entry — and the textbook explicitly states: "The pupil regulates and controls the amount of light entering the eye." The cornea is described as where refraction happens, not as the regulator of light entry. CBSE expects answer (b) Pupil based on this exact textbook line.