Which level of taxonomy has the fewest organisms?

Correct answer is "Species" is the lowest taxonomic rank, rank is the level of an organism or groups of organisms in taxonomy hierarchy.
Species is basic unit of biological classification which contains the fewest number of organisms and this makes it most specific.
Species which have the same ancestors are placed in one rank called genus.
Taxonomic ranks in order (from the biggest to the smallest) :)
-Domain
-Kingdom
-Phylum
-Class
-Order
-Family
-Genus
-Species

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