Final answer:
Stegosaurus is the extinct reptile that was once a land-dwelling, cold-blooded reptile.
Step-by-step explanation:
The reptile that is now extinct and was once a land-dwelling, cold-blooded reptile is option (d) Stegosaurus.
Stegosaurus is a genus of herbivorous dinosaur that lived during the Late Jurassic period, approximately 155 to 150 million years ago. It had a distinctive row of large plates along its back and had a small brain compared to its massive body size.
Stegosaurus is often depicted in popular culture and is one of the most well-known dinosaurs.