Answer:
Cephalochordata is the correct answer
Explanation:
Cephalochordata are the organisms which retain all the four characteristic features of chordates even in the adult stage of their life while in other chordates many of those features disappear in adult stage. In the early embryonic stages, all the chordates display those features at some stages of development but during due course of development some of these features disappear in all the chordates except for the members of phylum cephalochordata. One of the examples of cephalochordata is amphioxus.