"Whom does the boy like?"
Translation:Kogo lubi chłopiec?
The translation above says "Kogo lubi chłopiec?" so I assume that you ask why the translation "Kogo lubi 'ten' chłopiec?" was accepted.
In Polish we don't have the definitive nouns like "the boy", so if you mean some particular, one, special boy ("the boy") then the only way to say that in Polish is to say "ten chłopiec" - which translates best into "this boy".