Correct option is D
Explanation-
Living fossils are organisms that have remained morphologically similar to their ancient ancestors over millions of years. They show very little evolutionary change. They still exist today (extant).
Examples: Coelacanth (fish), ginkgo biloba (plant), horseshoe crab (Limulus), nautilus
Statement a : Living fossils are impressions of extant organisms in old rocks - INCORRECT
Impressions of organisms in rocks are called fossils, not living fossils. Living fossils are not rock impressions; they are real, living organisms.
Statement b : Living fossils show high morphological divergence from fossil records - INCORRECT
Living fossils actually show low divergence (i.e. very little difference) from their fossil ancestors. Their morphology has stayed almost the same for millions of years.
Statement c : Living fossils are always an evolutionary link between two classes of organisms - INCORRECT
Living fossils are not necessarily evolutionary links. Some may have unique evolutionary positions but many are just ancient forms that have survived. This is not all living fossils are transitional forms.
Statement d : Living fossils are organisms that have remained unchanged for millions of years - CORRECT
This is the correct definition of living fossils. They show morphological stability for a very long geological period.
So, the correct answer is option d - Living fossils are organisms that have remained unchanged for millions of years.
