The correct option is A Limulus
The horseshoe crab or Limulus polyphemus is a living fossil. These were the forms that are almost identical to the species which were present during the Triassic period 230MYA. Horseshoe crabs are not crabs in reality. They are related to the group of arachnids (spiders, scorpions, ticks and mites). They are the closest living relatives of the now extinct trilobites.
Thus, the correct answer is option A.