Sturgeon belongs to the class Actinopterygii, which includes ray-finned fish.
It depends on which species of sturgeon you are asking about. There are about 26 species of sturgeon all of which belong to the family Acipenseridae.
Insects belong to the class Insecta in the phylum Arthropoda.
Chickens belong to the class Aves, which includes all birds.
It belongs to the invertebrate class: Trematoda
No, humans belong to the class Mammalia, while bears belong to the class Mammalia as well. Both humans and bears are mammals, but they belong to different orders (Primates for humans and Carnivora for bears).