Answer: shark
Nekton is term which defines pelagic animals that can swim independently in the water currents. It includes three phylla of animals.
1. Vertebrates: These animals exhibit bones and cartilages. This includes species of bony and cartilaginous fishes such as sharks, reptiles like turtles, snakes and crocodiles.
2. Mollusks: This includes squids, octopus and scallops.
3. Crustaceans: This includes shrimps, crabs, and lobsters.
On the basis of above information, shark is the only nekton among the given examples.