Answer is B) One swims freely and the other often lives attached to a hard surface.
Nektons are marine aquatic actively-swimming organisms. Nektons propel in the water column freely either by swimming or other by means. Examples include vertebrates such as fish, whales, turtles, sharks and invertebrates include squads.
Benthos are the animals which are found at the bottom of the seafloor. These animals can be free moving forms near the ocean bed or attached to the seafloor. Unlike the nektons, benthos cannot swim in water. Benthos mainly includes echinoderms, crustaceans, molluscs, poriferans and annelids.
one would be wind another would be water density