The classification is in this order: Kingdom, Phylum, Class, Order, Family, Genus Species.
The kingdom is the largest group, it contains the largest number of organisms who are the least related.
The species is the smallest group, it contains the least number of organisms who are closely related.
As you go down, from "Kingdom", species become more closely related.
So species in the same family are MORE closely related than species in the same class.
The answer would be algae or phytoplankton
