Final answer:
Tardigrades are not included in the phylum Arthropoda; they belong to their own phylum, Tardigrada, making option b) tardigrada the correct answer.
Step-by-step explanation:
The question is asking us to identify which group is not included in the arthropods. The phylum Arthropoda includes several subphyla such as Chelicerata, Myriapoda, Hexapoda, and Crustacea. These subphyla represent a variety of arthropods with distinct characteristics, such as spiders, centipedes, insects, and crustaceans respectively.
Tardigrades, however, are not part of the Arthropoda phylum. They are instead part of their own phylum known as Tardigrada. Therefore, the correct answer to the question 'Which of the following groups is not included in arthropods?' is b) tardigrada.