Which of the following organisms is classified as a living fossil?

The term "living fossil" refers to a species that has remained relatively unchanged over long geological periods and closely resembles fossils of its ancestors. Cycas, a genus of cycads, is often considered a living fossil because it has existed since the Mesozoic era and shares many characteristics with its ancient relatives.

Cycads like Cycas display features believed to be similar to those of early seed-producing plants, thus showing very little evolution despite the extensive changes in the environment and other plant lineages around them. This lack of significant morphological change over millions of years contributes to their classification as living fossils.

While the other organisms listed also have noteworthy evolutionary histories—such as the coelacanth, which is another prominent example of a living fossil, having originally been known from the fossil record and discovered to still be alive in the 20th century—Cycas is validly recognized as a living fossil due to its persistence and morphological similarity to ancient plants.