Final Answer:
A lancelet is a fish-like marine invertebrate that belongs to the subphylum Cephalochordata, so the correct option is A.
Step-by-step explanation:
Chordates are a phylum of animals that share four key characteristics: a notochord, a dorsal nerve cord, pharyngeal slits, and a tail. Vertebrates are a subphylum of chordates that have a backbone made of bones or cartilage.
Lancelets, also known as amphioxus, are small, fish-like animals that live in marine sand. They have all four of the key characteristics of chordates, but they do not have a backbone.
Instead, they have a notochord, which is a flexible rod that runs along the length of their body.
Sea stars, frogs, lampreys, and sharks are all vertebrates. They have a backbone made of bones or cartilage, which gives them a more rigid body structure than lancelets.
Sea stars are echinoderms, frogs are amphibians, lampreys are jawless fish, and sharks are cartilaginous fish.
So the answer to the question is A lancelet.