Latveria is a land that exists only in the universe of Marvel Comics and first appeared in "Fantastic Four Annual #2", published in 1964. It is, supposedly, an isolated European country ruled by the fictional Supreme Lord Doctor Doom, located in the Banat region. It is surrounded by the Carpathian Mountains and borders Symkaria (home of Silver Sable) to the south. Its capital is Doomstadt. To be taken seriously as a country a fictional land would need a flag, a language and a currency:

-- The picture is of the Flag of Latveria, based on the design that appears on the cover of "Doom: The Emperor Returns #1"

-- The native language is close to modern German.

-- The currency is the Latverian Franc

Now for the other answer options:

-- Several countries other than the US use the US dollar as their official currency. At present (March 2020) these include El Salvador, Ecuador and Timor Leste.

-- The euro is the official currency of 19 of the 27 member states of the European Union.

-- The lats is the former currency of Latvia (a real country!)

More Info: en.wikipedia.org