Translation:They want hot milk.
Wow. I have been studying Chinese since 1985, and I don't consciously remember ever seeing the character 牠 (or 祂 for that matter, although I probably saw them at some point while studying 文言). However, looking them up on Wiktionary, the first one at least - with the animal radical - is cited in dictionaries as old as 辞海 (1915) and 康熙字典 (1716), as well as more recent ones. They certainly aren't modern usage, anyway! It's possible thay they were used in 文言 (literary Chinese) and have since fallen out of use.
You can see the entry from the 1716 Kangxi Zidian here: http://www.kangxizidian.com/kangxi/0697.gif
It's written in literary Chinese; it gives the pronunciation (using some other archaic forms of 'it') and says a couple of other thjngs I've forgotten how to read. :) I'd like to look into this further, but for now, I think it's safe to say they're pretty much archaic/obsolete. That doesn't mean you might not see them occasionally in modern writing, though.
I'm curious where you even found these, Alex!
Well, 她 alone means "she". «她们» means "they", but in written Chinese denotes a group of women. The aforementioned is a bit particular-- most people opt for the general 他们 (or even 它们) that can be used for all sexes. In speech, it all sounds like tāmen, so it remains ambiguous
The problem with forcing 她们 in a listening exercise, is that duolingo is actually teaching you poor habits. If it's not clear what gender the "they" is, you should always use 他们. If Duolingo wants to enforce a specific answer they should enforce the masculine "tā" so we learn good habits.
As it stands it just forces you to guess which tā they decided to use today, and arbitrarily marks you wrong when you pick the wrong gender half the time. It's really frustrating.
That should also be an acceptable answer, yes. But it's optional. And in this case, I don't think most people wouldn't use 的. Typically you add that when answering a question like 他们想要什么样的牛奶？or 他们想要那一种牛奶？But as a statement of fact, where you aren't really trying to emphasize the adjective, you wouldnt really use 的.