Hello!! Your best answer would be C - synapomorphy and homoplasy. Like any other animal/species, you can determine, so A is out completely. As for B, it means that ancestral characteristics/traits are shared from two or more taxa (biological group given a name). As for me, I do not think that is the best answer because they have always had a vertebral column, which is why they have bones and a vertebra. C is the best answer because synapomorphy means that there is a trait that arose from an ancestor and it is present now. Homoplasy means that there is a presence of different groups that a trait is not inherited from the common ancestor. This usually ties to convergent evolution, which fish is a part of the convergent evolution pool of species. I hope I helped!! Have a great day. :)