It is common for people to use the term "NATIONALITY" as their ethnicity or ethic background. However, in U.S. Immigration Law, nationality means citizenship. However citizenship can be confusing as well; dual or multiple citizenship is not uncommon in Europe. Depending on the law, citizenship can be gained lex sanguinis (by descent), lex soli (by birth) or by naturalization. So your mom could have citizenship in Bulgaria, Moldova and Armenia.
To keep it simple and practical, since she has an Armenian passport, that should be her NATIONALITY. Also it should be consistent. For example when she entered the DV lottery, what was the stated as her nationality? Goodluck
nationality = citizenship. Is she a bulgarian citizen? Nope. Then put Armenian.
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