What class of arthropods is characterized by having 4 pairs of legs?

The class of arthropods that is characterized by having four pairs of legs is Arachnida. This includes organisms such as spiders, scorpions, ticks, and mites. In this class, adults typically possess eight legs, a key distinguishing feature compared to other classes of arthropods.

Insecta is characterized by having three pairs of legs, totaling six legs in insects, which separates them from Arachnida. Crustacea, which includes crabs, lobsters, and shrimp, can have a varying number of legs, often more than eight, but they do not fit the definition of having exactly four pairs. Myriapoda, which includes centipedes and millipedes, typically have many legs but also do not conform to having four pairs. Thus, Arachnida is the only correct choice reflecting the defined characteristic of possessing four pairs of legs.