another possible solution is,, where will they keep the bottles of wine?
Well, the original sentence is passive, so there's nothing to tell you it's "they". I could be speaking about my own bottles.
When one sees the verb "essere," suspect the passive, è vero?
Yes, suspect; but unfortunately there isn't always a clear indicator :) An auxiliary that clearly implies the passive is venire: "dove verranno tenute le bottiglie di vino?".
why not "where will they have kept the bottles of wine"?
That would be “Dove avranno tenuto le bottiglie di vino?”
I wrote "where will the bottles of wine have been kept?" Is that not right because there was no avranno? Confusing!
2019-07-26 Not active, as Greg wrote, but future perfect, as in https://forum.duolingo.com/comment/10748934 . The difference is that the future perfect adds state, which the simple future lacks.
The sentence is passive but you made it active.