I think most country names are treated as grammatically singular, even if they don't look like it; "The United States", "The Philippines", "Trinidad and Tobago" are each the names given to a single country and you'd use singular verb agreement. (Unless you were specifically referring to their separate constituents: "Trinidad and Tobago are islands", or "The United States of America are Alabama, Alaska, Arkansas...")
It depends on the dialect of English, actually! Some lean towards plural, some lean towards singular. Both are technically correct if you're talking in the broad sense of English, but if you want to narrow it down by dialect then it'll start to differentiate.
In my dialect, for example, one would say are, because we lean plural. But I'm 99% certain a Brit would say is.
... and then, of course, there are dialects that don't conjugate the verbs as much (such as African-American Vernacular English - pretty sure they'd just use "be" in this case, but I only have surface knowledge there so I'm not sure), but those are rarer and in the broad sense of English, they aren't considered "correct" (even though, technically, they're a valid dialects, so they are, in fact, correct). It's a good guess to assume that the Duolingo answers are not going to include such dialects.
A nice explanation here on the difference between (The Kingdom of) the Netherlands and Holland. Perhaps they are referring to Aruba or Curacao in the sentence not Amsterdam or the Hague.