Which organism is an example of Arachnida?

Scorpions are classified within the class Arachnida, which is characterized by organisms that generally possess eight legs and two main body segments: the cephalothorax and the abdomen. This distinguishing feature sets arachnids apart from other arthropods. Scorpions also exhibit unique traits such as pedipalps, which can be used for grasping prey, and a segmented tail that often ends with a stinger.

The other choices represent different classes of arthropods. Crayfish belong to the class Malacostraca within the phylum Crustacea and are characterized by having ten limbs. Pillbugs, or roly-polies, are also crustaceans, specifically isopods, that exhibit a segmented body and seven pairs of legs. Millipedes belong to the class Diplopoda, known for their elongated bodies with numerous segments, each typically bearing two pairs of legs. Thus, scorpions correctly exemplify organisms in the Arachnida class.