Which one is a true fish?

Concept: A true fish belongs to the phylum Chordata and Vertebrata, so they have a backbone. They've had notochord during embryonic development which is now replaced by the backbone. They belong to class Pisces and have a single sheltered heart. They breathe through gills and have fins for thrust
Dogfish is a common name for Scoliodon. It's a cartilaginous fish, classified under class Chondrichthyes in class Pisces. It's a true fish that comes under the phylum Chordata.

Hence, the correct answer is option C.