The correct option is C Insects
We have currently identified and classified about 1.5 million eukaryotic species. Of the species identified so far, about 950,000 species of them are insects. Therefore insects form the most diverse taxa, close to 60% of all species. Only eukaryotic species are accounted for in biodiversity, because species boundaries are ill-defined in prokaryotes. Therefore viruses have not been accounted for here. There are about 350,000 beetles that have been identified so far, that is close to 40% of all species. Since close to 60% of the pie-chart has been indicated here under this group, the missing taxon here is insects.