Final answer:
Crabs and scorpions are both arthropods; they share characteristics like an exoskeleton made of chitin and jointed appendages. Jellyfish, which have stinging cells called nematocysts, belong to a different group, the phylum Cnidaria.
Step-by-step explanation:
When comparing the common characteristics of a crab, scorpion, and jellyfish to determine which two animals belong to the same group, option D is correct: Crab and Scorpion. Both crabs and scorpions are arthropods which share key characteristics such as an exoskeleton made principally of chitin, a tough polysaccharide, and the presence of jointed appendages. Crabs, as crustaceans, and scorpions, as part of the class Arachnida, both fall under the larger phylum Arthropoda. On the other hand, jellyfish belong to the phylum Cnidaria, which is characterized by aquatic organisms with stinging cells known as nematocysts, and they do not share the arthropod features of an exoskeleton and jointed appendages.