Aquatic organisms are broadly classified based on their mode of life and movement in water. - Plankton are organisms (plants - phytoplankton, and animals - zooplankton) that float or drift in the water, largely dependent on currents for movement. - Benthos are organisms that live on or in the bottom substrate of an aquatic environment. - Nekton (or Necton) are organisms that are active swimmers and can move independently of water currents. This includes fish, marine mammals, squid, etc. Options A, C, and D do not accurately describe Necton. Floating organisms are plankton. Animals associated with aquatic plants could be either plankton, benthos, or nekton depending on their mobility. Sub-merged plants are producers, not necessarily defined by their mobility in this classification.