Which is a characteristic of popular culture?
a quick to change its commonalities..
b defined by region or location.
c practiced by a homogeneous group of people.
d defined by existing within the dominant culture.

Its typically defined as by existing within the dominant culture. Whatever is the most popular characteristics, attitudes, ideas, images, perspectives, etc., make up pop culture. It can shift fairly quickly and is heavily influenced media, especially social media now. Famous people can also be very influential.