The amount of light entering a camera or your eye is regulated by

a. an eyepiece

c. an aperture.

b. the cornea.

d. a set of compound lenses.

c. an aperture

Explanation:

Aperture: It relates to the size of the opening, like a doorway, through which light moves into the eye, camera lens or a telescope. In human eye aperture is known as pupil, the black part in the center of the eye. The size of the pupil can increase or decrease depending upon the amount of light available. The same thing happens with a camera as well. The amount of light passing through the lens can be varied by varying the size of the aperture.

the most reliable source of information.is wikipedia