Final answer:
At the end of the Cretaceous, the mosasaurs, along with the dinosaurs and many other species, went extinct due to a large meteor impact. This event cleared ecological niches for the survivors to evolve, leading to the diversity of modern mammals in the subsequent Cenozoic Era.
Step-by-step explanation:
Animals that became extinct at the end of the Cretaceous include mosasaurs. The end of the Cretaceous period, about 65 million years ago, saw a massive extinction event that wiped out the dinosaurs and many other forms of life, such as various aquatic animals and the ammonites. This event is well understood to have been caused by a catastrophic meteor impact that resulted in a sharp decline in biodiversity. Though this extinction event had profound effects, it did not significantly impact placental mammals, birds, lizards, snakes, turtles, crocodiles, fishes, and plants, which all fared relatively well in the aftermath.
The end-Cretaceous extinction allowed for the evolution of modern mammals, as many niches previously occupied by dinosaurs became available. This diversification and adaptation led to the flourishing of mammal species in the ensuing Cenozoic Era. The other options presented in the question—whales, forams, and cycads—did not have documented mass extinctions at the end of the Cretaceous period; although, forams did suffer significant losses.