Select the invertebrate.

 - rockfish
 - green frog
 - Canadian lynx
 - monarch butterfly
A monarch butterfly is an insect. Like other insects, a monarch butterfly is an invertebrate. It does not have a backbone. It has an exoskeleton.
A rockfish is a fish. Like other fish, a rockfish is a vertebrate. It has a backbone.
A Canadian lynx is a mammal. Like other mammals, a Canadian lynx is a vertebrate. It has a backbone.
A green frog is an amphibian. Like other amphibians, a green frog is a vertebrate. It has a backbone.