The cephalothorax is a fusion of
a. the head and tail region
b. the abdomen and thorax region
c. the head and thorax region
d. none of these, it is not fused

The answer is c. the head and thorax region.

The word cephalothorax is derived from two Greek words - cephalon, meaning head, and thorax. So, it is a fusion of these two body parts, and it is characteristic for chelicerates (scorpions, spiders,...) and crustaceans (crabs, lobsters, shrimps,...) . Cephalothorax is unsegmented and rigid and it is easy to differ it from the abdomen.