Correct option is B
Explanation-
A. Kallimodon → X (Fossil)
About: Kallimodon is an extinct genus of reptile from the Late Triassic period.
Status: It is only known from fossil records.
Conclusion: No living representatives → it is a true fossil.
B. Tuatara → Y (Living fossil)
About: The tuatara is a reptile native to New Zealand, belonging to the order Rhynchocephalia.
Status: Although all other members of this ancient order are extinct, tuataras are still alive today.
They have remained morphologically similar to their ancient ancestors. Represent a lineage that dates back over 200 million years.
Conclusion: It is a living fossil.
C. Xiphosura → Y (Living fossil)
About: Xiphosura includes horseshoe crabs, which are marine arthropods.
Status: Existing species (like Limulus polyphemus) have survived virtually unchanged for over 450 million years.
Their body structure has changed very little over geological time. They coexist with fossils that look nearly identical.
Conclusion: They are living fossils.
D. Ankylosphenodon → X (Fossil)
About: Ankylosphenodon is an extinct genus of reptile from the Early Cretaceous period.
Status: Known only from fossil specimens; no living descendants.
Conclusion: It is a fossil, not a living organism.
Final Correct Pairing:
A, D → X (Fossil)
B, C → Y (Living fossil)
Correct answer is option b - A, D are X; B, C are Y
