Question

Flatworms crabs and starfish can all be classified as

Answer 1

Flatworms crabs and starfish can all be classified as invertebrates. 

Answer 2

They are classified as Invertebrates. 
Answer choice B. Invertebrates is correct.
Also here is a bit more information which is a list of invertebrate species..
1. Clams,
2. Squids,
3. Crustaceans,
4. Arthropods,
5. Insects,
6. Worms,
7. Jellyfish,

I hope this helps!