Iris regulates the amount of light entering the eye by controlling the size of the _______.

The correct option is B pupil
Pupil is an aperture at the centre of iris. The size of this aperture increases and decreases depending upon the intensity of light. In dim light the pupil is dilated, while in bright light it is constricted. The size of the pupil is regulated by the activity of radial and circular muscles of iris. Contraction of circular muscles decreases the size of the pupil and contraction of radial muscles increases the size of the pupil.