it is grammatically singular in English
except among extreme libertarians and the Strict Constructionists. True, in the popular press the phrase "these united states are..." had been mostly supplanted by the phrase "the united states is..." by around 1840, more than 20 years before the contest between States' Rights and Strong Central Government had been settled at the point of a bayonet; nevertheless, it is not grammatically incorrect to say "these united states are..." (just not common anymore)
In Canada, we would say 'The United States is'/'the government is'/'everyone is' because they are seen as a single entity. With 'everyone', the syllable 'one' is a clue why we use 'is', yet we say 'all are'. Another clue to the use of 'is' here is what follows the verb...it is A great country...singular. On another note, if this wasn't multiple choice, I would have assumed it was referring to size, not greatness. Is there a way to tell the difference?
Before the Civil War, the United States was often referred to in the plural. "The United States are..." "The United States have...", etc. It was more common when the Federal government was practically nonexistent. The United States ARE a confederation of sovereign states, after all.
American English typically treats collective nouns as singular rather than plural - "everyone is", not "everyone are". "The government is" never "the government are". Interesting to hear about the way British English deals with this! As the U.S. is a single nation (pays), and "grand" modifies "pays", that clause then describing "les etats-unis", I can see how this ends up with the weird disagreements. I do want to know how else this is dealt with in French!
Usually but not always. This may help:
(scroll down to the section on "Position of French Adjectives")
My problem with this sentence is that the English I am asked to translate said "the US is a GREAT country," which i took to mean powerful or strong. If I wanted to talk about its size I would not use "great" by itself, I'd use large or big. And if I'm saying it's a powerful country, the adjective would go after the noun: un pays grand (because grand is the only option, though it should be puissant or fort). They should not use Great as the translation, it's too ambiguous. You mean big? Say big.