Hmmm, I did get it right but I didn't really like the question. Or rather, the answer options; the question itself is great.

I'd say that both "drop the DB" and "you can't" are correct. And if you really want to split hairs, you could even say that "you can't" is the only correct option, because the question was on removing the FG from the DB; dropping the DB removes the entire DB, with all its FGs, it removes the FG from existence but not from the DB.