The correct answer is option D, 520 Million Years Ago
Reason -
The first invertebrates occurred in the oceans in the form of soft body animals which are covered by shell. These organisms appeared in the water bodies some 5 billion years ago (500 million years ago). The other species include the arthopods (insects), mollusk(snails and octopus) , annelids (Earthworm and leeches) etc. also evolved some 490 million years ago.