Awkward English translation. More common would be, "To whom do they belong?"
Wouldn't "to whom" be "A cui?"
"Whom ya gonna call?" Really!?
"To whom they can belong?"
Hello? D, why are you marking it wrong? It's fully acceptable.
It is quite correct in english to add to after belong
To whom can they belong?! Are you serious duolingo?! If anyone still talks like that they are ridiculous. This word needs to go away and die :)