Final answer:
Insecta is distinguished from the other arthropod classes by the three body regions: head, thorax, and abdomen.
Step-by-step explanation:
Insecta is distinguished from the other arthropod classes by the three body regions: head, thorax, and abdomen. The abdomen of an insect contains most of the internal organs and is responsible for digestion, excretion, and reproduction.
Unlike insects, other arthropods such as spiders and crustaceans may have different body regions, such as a cephalothorax and abdomen or a head and trunk.
Therefore, the correct answer to the question is Abdomen.