The correct option is
C Merostomata
The class Merostomata is an ancient class of animals. It evolved in the early Cambrian (570 million years ago) and were a dominant life-form in the seas and oceans of the world until the Permian 310 million years ago. Now they are a very small group which consists of only 4 species in 3 genera and one order (or subclass)
Xiphosura.
Limulus is a genus of horseshoe crab from subclass
Xiphosura So, the correct answer is 'Merostomata'