That sounds like an incomplete sentence (without context). I see What kind of [missing] are the pants?; how do you come up with the category? If the question was only What are the pants? then ok, then I don't need to specify any group. I tried to translate it as What kind of pants is it? which is apparently wrong.
It is not an incomplete sentence.
Kia estas is what one always asks to know what kind of, which kind of something, something is (the interrogative and relative correlative of kind).
I remember Evildea explained this very well in one of his videos.
Tio estas elfo
Kia estas la elfo?
Ĝi estas nokta elfo.
Sometimes it helps to not refer to another language when learning a new one. New concepts, patterns, expressions or even vocabulary might come up. Sure, you want to learn how to say different things, or translate them, but if one doesn't have a good grasp of how a new language works, it's generally a idea of saying "that sounds strange".
Why can't it be "what are the trousers like?" Why does it have to be specifically a kind of trousers?
Besides... how does "What sort the trousers like?" work? I mean, English is not my first language so I'm not sure, but that answer Duolingo suggests sounds really weird to me.