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

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 =)

This is what i found on google “phytoplankton, tiny organisms that live in the ocean, are autotrophs. some types of bacteria are autotrophs. most autotrophs use a process called photosynthesis to make their food.” i hope this you for what your looking for