Answer:
The correct answer will be diatoms.
Explanation:
Diatoms are photosynthesizing micro-algae found in both marine and freshwater usually hard and rough in appearance due to deposition of silica in their cell walls.
They live in both photic and planktic region of the ocean and in both unicellular to colonial forms and form the major part of the planktons.
Their large number of species counts for about 20,000 and new species being discovered every year and their reproduction through simple cell division makes the most numerous unicellular algae in the oceans
Thus, diatoms is the correct answer.
