It should be "Are your pants made of cotton fabric?"
I put "cloth" instead of "fabric" and got it wrong. The meaning is the same!
Cloth and fabric are similar but not the same. Cloth describes an actual piece of fabric of a certain size, fabric describes the material and has no size, its a generic term
Regionally or archaically cloth can also be a synonym for fabric. See for example Collins definition versus Merriam Webster definition.
I wrote trousers, that was not correct. Duolingo says it must be pants. Same no ?
Depends. In English pants is the same as underpants in American English pants is the same as trousers.