The answer is "they have to go", but "they need to go" is wrong? Aren't these answers the same?
Native-speaker here: "They need to go" should be accepted. I've submitted a report to duolingo.
I think this is interesting. When teaching English I do talk about in some small way, must, need to and have to are just slightly different in use. But it would be very hard to say if the English differences are consistent across different native English speakers and consistent with another language's use. Need to for me is personal, must is moral obligation, have to is physical reason. So I need to meet my boss because I want to discuss my poor salary, I must meet my boss to discuss the progress of the project, I have to meet my boss because he asked for a meeting. But much overlap, in duolingo all 3 should be ok. I think.