Which has the largest number of species? 

A)  insects and microorganisms 
B)  microorganisms and fungi 
C)  amphibians and reptiles 
D)  insects and fungi

Insects and microorganisms are the two largest groups in terms of species diversity. Insects alone account for over 1 million species, while microorganisms (including bacteria and archaea) are estimated to comprise around 5 million species. Fungi and amphibians/reptiles have significantly fewer species in comparison, making choices B and C incorrect. Insects and fungi may have a relatively high number of species, but not as high as the combination of insects and microorganisms.