Duolingo prefers "destes" for "these" as they would be near the speaker. "desses" is "those" for duolingo as they are near the listener. "daqueles is "those(, over there)" as they are not near either person. The confusion comes from when the listener is sitting right next to the speaker and you would be tempted to say 'these", but for duolingo assume that the listener is not so close to the speaker. It is not a problem, because you could always use "destes" if you want to say "these".