The correct option is Crab because it belongs to the class Malacostraca within the phylum Arthropoda, specifically under the category of crustaceans. In contrast, the other three organisms likely belong to classes that are either from a different phylum or a group of organisms that do not share the distinct morphological and ecological characteristics of crustaceans, such as insects or arachnids. This distinction illustrates the diversity within the animal kingdom and highlights the importance of taxonomic classification in understanding evolutionary relationships among organisms, a key concept in UPSC Prelims GS Paper I.