Translation:The cat is light.
While lightweight can mean something is light, it often means light in comparison to other similar objects.
It also can mean if something is useless/trivial. "The newspaper is lightweight." means that it doesn't cover anything important; it doesn't mean that the newspaper is actually physically light.
because it's not the right answer and it can not be. You wrote: the cat's light and that mean the light of the cat which is in Korean language 고양이의 가벼 so 가벼 in your sentence is not playing the role of the adjective because you didn't describe the cat but you talked about something existing in it, I am not an expert it's just what I understood if I am mistaken, I hope someone could explain better.