The answer is c because the dinosaurs were declining by the end of the cretaceous so the meteor/comet was the fatal blow for the dinosaurs.

In addition to your answer, according to history.com:

"The Cretaceous-Tertiary extinction event, or the K-T event, is the name given to the die-off of thedinosaurs and other species that took place some 65.5 million years ago. For many years, paleontologists believed this event was caused by climate and geological changes that interrupted the dinosaurs' food supply."