Question

Horseshoe crabs are one of the only five living species of arthropods class

Answer 1

C. Arachnida.
hope it's correct.

Answer 2

Answer:

A.  merostomata

Horseshoe crabs are merostomata class

Explanation:

Merostomata is one of the five class of the phylum arthropoda. Horseshoe crabs are not crustaceans. They are the only living representative of a group that flourished successful over 300 million years ago on the earth. They have lived in the coasts of North America and Southeast Asia.

Taxonomy:

PHYLUM:  arthropoda

SUBPHYLUM: chelicerata

CLASS: merostomata

ORDERS: xiphosura

FAMILIES: limulidae