Final answer:
The richest and most detailed fossil record for late Maastrichtian dinosaurs comes from western North America, which has been extensively studied and has provided significant insight into dinosaur distribution and their extinction at the end of the Mesozoic era. Therefore, the correct option is C.
Step-by-step explanation:
The detailed pattern of the distribution in geologic time of the last dinosaurs can be best observed in western North America. This region yields an abundance of late Maastrichtian dinosaur fossils and has been intensely studied, providing insight into the lives and eventual extinction of these ancient creatures. It was during the Mesozoic era, specifically at the end of the Cretaceous period, that the age of dinosaurs came to an abrupt end, approximately 65 million years ago. This coincides with the impact of a large asteroid, believed to be a significant factor contributing to the mass extinction event, alongside environmental changes brought about by volcanic eruptions and shifts in plant life and climate.