The correct answer to this open question is the following.
Although there are no options attached we can say the following.
"The habit of thinking of a people is an essential part of their culture". This observation explains the relationship between different cultures and corresponding civilizations because it is true that people, humans, are the most important part of a culture, of a nation. No people, no society, and their institutions. As simple as that.
The diversity of the people is what makes this life so wonderful and worth living. From the minds of people, evolution has been possible. Since our primitive version in the caves, until the great technology we have today that allow us to connect instantly through the internet, travel to space, explore planet Mars, improving the human's life span, and more.
So yes, people are an essential part of culture because they live that culture through traditions, history, and belief systems.