The correct answer to the question is option 2, Ginkgo. Ginkgo is commonly referred to as the living fossil because it is one of the oldest living tree species on Earth. It is believed to have existed for over 200 million years, with fossils dating back to the time of the dinosaurs.

Option 1, Cycas, is also an ancient plant species, but it is not typically referred to as a living fossil. Cycas is a type of cycad, which appeared around 280 million years ago. While cycads are considered primitive, they are not as old as the Ginkgo.

Option 3, Taxus, refers to the yew tree, which is not a living fossil. Yew trees have been around for quite a long time, reaching back to at least the Cretaceous period. However, they are not as ancient as Ginkgo.

Option 4, Williamsonia, is not a living plant at all. It is an extinct genus of plants that existed in the Late Triassic period, over 200 million years ago. Therefore, it is not the living fossil referred to in the question.

In conclusion, Ginkgo is the correct answer as it is commonly known as the living fossil.