Correct Option
Crab belongs to the class Crustacea within the phylum Arthropoda. Organisms in the class Crustacea are typically aquatic, possess two pairs of antennae, and commonly have five pairs of legs. This classification and these characteristics differentiate the crab from the other three organisms listed.
Incorrect Options
Mite, Scorpion, and Spider all belong to the class Arachnida, which is also a class within the phylum Arthropoda. Arachnids are primarily terrestrial, characterized by having four pairs of legs and no antennae. Since these three organisms share the common class Arachnida, they form a distinct group from the crab, which belongs to the class Crustacea. Therefore, options (b), (c), and (d) are incorrect because they belong to the same class as each other, unlike the crab.