The statement that is true about scientists in relation to pliosaurs is:
They learned most of what we know about pliosaurs by examining fossils.
Scientists primarily study fossils to understand the biology, behavior, and ecology of extinct species, including pliosaurs. Fossils provide valuable information about their anatomy, habitat, and how they might have lived. The other statements either make overly definitive claims or misrepresent the current understanding of pliosaurs. For instance, there are still ongoing discussions and research about the causes of extinction for many prehistoric species, including pliosaurs.