Answer: b. Necton
Step-by-step explanation:
Necton is the term used to refer to aquatic organisms that are able to swim against the current of the water they are in. For this reason they are called "actively swimming" organisms. Aquatic plants that are unable to move against the current are called Plankton.
Fish, whales and dolphins are able to move against the current and so are classified as Necton. It is worthy of note however, that some organisms are able to transition from Plankton to Necton as they mature.