Why Do the Authors of Your Text Use the Word

Why do the authors of your text use the word "domain" when talking about "the Russian Domain"?

A) "Russian Domain" is an official name, reflecting the continuing federal relationship between the countries in the region.
B) The term suggests persisting Russian influence within the region.
C) The term is the name of the free trade zone now evolving in the region.
D) Domain refers to a kingdom.
E) Domain refers to a federal subdivision.