"Which ones do they like?"
Translation:Lesquels aiment-ils ?
I'm no expert, but I think you can decide based on the gender, which will usually be provided contextually. Lesquels is masculine; lesquelles is feminine. So you could have "lesquels aiment-ils" or "lesquelles aiment-elles" for the masculine plural or feminine plural, respectively.
Although you are righr in saying that the choice is based on gender, it not the gender of the subject which determines it: you could have "lesquelles aiment-ils?" if the lesquelles is referring to something feminine, eg 'which girls'. If no context is provided, you should be able to use either (although the French typically default to the masculine form)
I'm not entirely sure. However, there would be no hyphen between ils and aiment in your sentence anyway as it is not an inversion. Only use hyphens when inverting (e.g. aiment-ils). To be safe, I would use inversion after lequel and it's variants (laquelle, lesquels etc), and 'lesquels aiment-ils' is of course what Duolingo recommends.
The "-t-" has the purpose of avoiding two vowel sounds in a sequence. If you have "Il aime lesquels?" and want to invert it, you have "Lesquels aime-t-il?" to avoid pronouncing the "e" in "aime" and the "i" in "il" in a row. In the case of the plural subject ("Lesquels aiment-ils?") you don't have vowels in a sequence between words, so you don't need the "-t-". I hope this helps...