The correct answer is option 4, "Nekton." Free swimming macroscopic animals in an aquatic environment are referred to as nekton. Nekton includes organisms such as fish, turtles, dolphins, and seals that are capable of actively swimming and moving against the current in the water. These organisms have the ability to control their vertical and horizontal movement and are not passively carried by the water currents.

Option 1, "Plankton," refers to organisms that are carried by the water currents and are unable to swim against them. Plankton includes both microscopic organisms (phytoplankton) and small animals (zooplankton) that drift in the water column.

Option 2, "Periphyton," refers to organisms that attach to surfaces in the water and grow as biofilms. Periphyton includes algae, bacteria, and small invertebrates that live on rocks, plants, or other submerged structures.

Option 3, "Benthos," refers to organisms that live on or in the bottom sediment of the aquatic environment. Benthos includes organisms such as worms, clams, and snails that are adapted to life in the sediments.

While the provided answer is generally correct, it is important to note that nekton