Step 1: Identify the Living Fossil Reptile:
The reptile known as a "living fossil" is the **Tuatara** (\Sphenodon punctatus). It is the sole surviving member of an ancient order of reptiles that flourished 200 million years ago.
Step 2: Identify the Order:
\Sphenodon belongs to the order **Rhynchocephalia**.
Step 3: Other Options:
(A) Chelonia (Turtles/Tortoises).
(B) Crocodilia (Crocodiles).
(C) Squamata (Lizards and Snakes).
Step 4: Final Answer:
Option (D) is correct.