Final answer:
Trilobita were the most abundant arthropods during the Cambrian period, with a vast presence in the fossil record.
Step-by-step explanation:
Members of the subphylum Trilobita were abundant during the Cambrian period. Trilobites were a group of extinct marine arthropods that were particularly prominent during the Cambrian, as evidenced by their diverse and abundant fossils found from this era. While the options provided in the question (Hexapoda, Myriapoda, Chelicerata, Crustacea) include subphyla with members that existed during the Cambrian period, none were as abundant or characteristic of the period as the trilobites, which are now extinct.