Question

Which part of the eye is a muscle that regulates the size of the pupil?

Answer 1

The iris sphincter muscle, also known as the papillary sphincter or sphincter papillae, is a muscle located in the colored part of the eye called the iris.Explanation:

Answer 2

It is the Iris which is a muscular structure and regulates the size of pupil according to the incoming intensity of light.

Hope this helps mate =)