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The correct answer identifies the Phylum Chordata as the taxonomic group that encompasses both fish and amphibians. This phylum is characterized by specific features that all members share at some stage of their development: a notochord (a flexible, rod-like structure supporting the body), a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

Fish and amphibians are both classified under this phylum because they exhibit these chordate characteristics during their life cycles. Fish, for example, are primarily aquatic vertebrates that possess gills and scales, while amphibians, which include frogs and salamanders, typically have a dual life stage—living both in water and on land, exhibiting characteristics such as moist skin to assist in respiration.

In contrast, the other taxonomic groups listed do not include both fish and amphibians. Phylum Arthropoda includes invertebrates such as insects, arachnids, and crustaceans, which do not share the chordate traits. Phylum Mollusca consists of soft-bodied animals, including snails and octopuses, whereas Phylum Annelida encompasses segmented worms like earthworms and leeches, which also lack the defining characteristics of chordates.