The correct answer is option 3 - Gingko. This plant is referred to as a living fossil because it is considered to be one of the oldest surviving plant species on Earth.

Let`s take a closer look at the other options to understand why they are not correct:

Option 1 - Ephedra: While Ephedra is an ancient plant and has been around for millions of years, it is not commonly referred to as a living fossil. It is known for its medicinal uses and unique appearance.

Option 2 - Cycas: Cycas is another ancient plant that has been around for millions of years. It is often referred to as a living fossil because of its longevity and relatively unchanged appearance over time. However, in this question, Gingko is the more specific answer.

Option 4 - Adiantum: Adiantum, also known as the maidenhair fern, is not referred to as a living fossil. It is a common fern found in various parts of the world, but it does not have the same ancient lineage as Gingko or Cycas.

In conclusion, while options 1, 2, and 4 have some historical significance, it is option 3 - Gingko - that is specifically referred to as a living fossil due to