Among the following organisms, which one does not belong to the class of the other three?

Crab is the odd one out. Crabs belong to subphylum Crustacea (class Malacostraca), along with prawns, lobsters and barnacles. Mites, scorpions and spiders all belong to class Arachnida, marked by four pairs of walking legs and no antennae.

Why the others are wrong: the question asks for the one that does not share the class of the rest, and B, C and D are all arachnids.

• Mite (B): Order Acari, a group of Arachnida, often tiny and sometimes parasitic.
• Scorpion (C): Order Scorpiones, with pedipalps and a sting in the tail.
• Spider (D): Order Araneae, with silk glands and spinnerets.

Core concept: classification of arthropods into Crustacea, Arachnida, Insecta and Myriapoda. Mains angle: arthropods matter for public health, such as mites in scrub typhus and ticks in Kyasanur Forest Disease, and for agriculture as pests and predators.

Trap: aspirants see that all four are arthropods and pick at random, or assume spiders and scorpions are insects. Ask which class the animal belongs to, not which phylum.