Solution

A living fossil is a living species of organism that closely resembles the species. Limulus is a genus of horseshoe crab with one extent species, the Atlantic horseshoe crab (Limulus polyphemus). The horseshoe crab is considered a living fossil because it is one of the very few animals that is still around that we can find fossils of them dating back 500 million years.
So, the correct answer is 'Limulus'