"Who(m) do you ask" is also correct. (I don't know if DL accepts it, but it should.)
Duo accepted it from me and offered "Who are you asking" as another correct solution (which is colloquial, not formal, English).
NourAkel, Josee, your English is off. It can't be "whom is asking". It must be "who is asking".
who is asking = vem frågar
who is asking you = vem frågar dig
who(m) are you asking = vem frågar du