"It is the capital."
Translation:Es ist die Hauptstadt.
In English (or at least American English) , the correct word is "capitol" , not "capital" . But Duo marked me incorrect on a related lesson where i wrote "capitol" when translating "Hauptstadt " .See https://www.grammarly.com/blog/capital-vs-capitol/
Even looking at your link, 'capital' is the correct choice. This is what your link says regarding the matter:
"Capital can be a noun or an adjective. Capital can refer to uppercase letters, accumulated wealth, or the city that serves as the seat of a country’s or state’s government."
We are speaking of a city, so it is capital.