The size of pupil is controlled by the _____.

Pupil size in the human eye is governed by an antagonistic pair of smooth muscles housed in a coloured, muscular ring positioned between the cornea and the lens: a circular set that constricts the opening and a radial set that dilates it, together widening or narrowing the pupil to control how much light reaches the retina.

That muscular ring is the iris; in this item, it is the structure that directly changes the pupil's diameter — widening it in dim light and narrowing it in bright light. The lens, cornea, and retina each perform a different job in vision, but none of them physically resizes the pupil.

• Lens: sits just behind the pupil and changes its own curvature (accommodation) to focus incoming light onto the retina; it has no muscle fibres that can widen or narrow the pupil itself.

• Cornea: the transparent, curved outer covering at the very front of the eye; it bends light as a fixed refractive surface and contains no muscular tissue to alter the pupil's size.

• Retina: the innermost, light-sensitive layer at the back of the eye that converts the light already admitted through the pupil into nerve signals via rods and cones; it responds to light rather than regulating how much enters.

So the size of the pupil is governed by the iris's sphincter and dilator muscles, which is why this is the correct choice.