Final answer:
The most species-rich type of animal among the given options is B) arthropods, which includes a wide variety of insects, as well as creatures like crabs and lobsters, making them the most diverse group in the animal kingdom.
Step-by-step explanation:
The student is asking about the type of animal that has the most species. Among the options A) chordates, B) arthropods, C) molluscs, and D) nematodes, the correct answer is B) arthropods. This group, which includes crabs, lobsters, and a wide variety of insects, is the most diverse in terms of species count.
With their jointed appendages, arthropods constitute the largest phylum in the animal kingdom and vastly outnumber the species found in the other options listed. While vertebrates, part of the phylum Chordata, are diverse, they do not come close to the sheer number of species found in Arthropoda.