Yeah, I know. But Duolingo could, at the very least, include the article in the "sentence".
Marketing is rarely used with an article, mostly to emphasize, I'd say. Just in case you're still looking for it, it's 'das'. ngram Viewer, if accurate, supports this assumption.
Sorry, I think you're missing the point of my comment-- as the word is identical in both English in German, the least they could do is put it into a full sentence, so we're quizzed on grammar and context. As it is, "Marketing?" is a rather poor sentence.