Why do people often treat the idea of a nation and the idea of a culture as synonymous?

There are basic similarities among people within the same national boundaries.

Explanation:

The definition of a nation is a body of people organized on the basis of common language, culture, ethnicity territory, etc.  Traditionally common culture is an important element to organized people into a nation. But the nation has other elements too which separate the meaning of nation and culture. And it should be noted here that today a nation is a political entity and the element of common culture remains a part of it.

according to classical republican political theory, which democratic republics would function best?

a.   small republics, with people who had similar interests

b.   large republics, with people who had competing interests

c.   republics made up entirely of landowners

d.   republics made up entirely of servants