Aregion based on common properties such as population is called a(n)?

A region based on common properties such as population is called 
a formal region

Formal region.

Explanation:

A formal region is an area inhabited by the people who have one or more characters in common and some have a distinct boundary which makes them easy to identify such as states as that of the U.S, Canada and the Africa, etc. Theses proprieties can be a language and the behavior or the way of perceiving things. Expressed by a community at large and is thus a group or collection of the traits and the standard values of the place.

The sun's parallax would be 180 degrees, greater than which you can't get.  so a. would be your anwser