Kia = what kind of
Kia estas la pantalono? = What kind (of trousers) are the trousers?
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.
"Pants" is grammatically plural in English. If you had written, "What kind of pants are they?", then it would probably have been accepted.
Couldn't "How are the pants" work? Couldn't the answer be, e.g., "La pantalono estas malvarma", since "kia" asks for an adjective?
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.