"Are you answering?"
There is only one correct answer.
Svar = answer (noun form, example: I have the answer.)
Svarar = answer (verb form, example: Are you answering?)
It can't be "Svar du?" because Svar is not the verb form, Svarar is. If "Svarar du?" had been an option it would have also been correct, but at this time only the options, "Svar du?", "Var ni?" and, "Svarar ni?" are given so there's only one correct answer: "Svarar ni?"