The correct option is B Merostomata
Limulus is a genus of horseshoe crab, with one extant species. These horseshoe crabs belong to the class Merostomata which are an ancient class of animals who evolved in the early Cambrian and were a dominant life-form in the seas and oceans of the world until the Permian 310MYA. Now, they are a very small group consisting of only 4 species in 3 genera and one order or subclass Xiphosura.
Therefore, the correct answer is option B.