True, but "which of them" is not necessarily singular in English, and "which of them" just presumes a group to which the speaker refers (as opposed to "who" in general). Though I admit 1. the verb should probably then be "drink" not "drinks" (unless it is known to be just one of a group, which is even more context-specific), and 2. I admit that it isn't the first phrasing that comes to mind.
Questions in spanish often switch the subject and verb around. Ex. Bebe usted el agua? Bebe el agua usted? Both mean "Do you drink the water." Can the same word switching be done with the above Duolingo sentence? Ex, "Beben quienes cafe con azucar"? Or Beben cafe con azucar quienes? Gracias