Name an example of a plankton organism.

Plankton are small organisms that drift in the ocean currents, and they can be classified into two main types: phytoplankton (plant-like organisms) and zooplankton (animal-like organisms). Diatoms, which are a type of phytoplankton, are single-celled algae with intricate silica shells. They play a crucial role in aquatic ecosystems, serving as a primary food source for many marine species and contributing significantly to global photosynthesis and oxygen production.

The other organisms listed do not fit the criteria for plankton. An octopus is a benthic organism and typically resides on the seafloor; coral is a colonial organism that forms structures on the ocean floor; and a shark is a large, free-swimming predator in the ocean. These organisms, while all vital to marine ecosystems, do not drift in the water column like plankton does, making diatoms the correct answer as they exemplify true planktonic life forms.