Final answer:
The group of arthropods that has a body divided into a cephalothorax and an abdomen is spiders, which are part of the subphylum Chelicerata.
Step-by-step explanation:
The group of arthropods that joins segments into a cephalothorax and abdomen is spiders (3). This is characteristic of the subphylum Chelicerata, which includes not only spiders but also mites, scorpions, and other similar organisms. In contrast, insects (Hexapoda) have a body divided into a head, thorax, and abdomen, while crustaceans usually have a distinct head and thorax, although some have a cephalothorax. Centipedes and millipedes (Myriapoda) have a series of body segments that are not fused into a cephalothorax.
Arthropods are known for their jointed appendages and segmented bodies, which have allowed them to adapt to various environments on Earth, making them one of the most successful animal phyla. The fusion of the segments into a cephalothorax provides certain advantages, such as a unified and protective structure for vital sensory and feeding appendages.