The Cenozoic Era started about 66 million years ago with the mass extinction of dinosaurs. Because of this, D could not have been true during the Cenozoic Era.
Trilobites and shellfish are both far more primitive than dinosaurs, even, and would also have evolved before the Cenozoic.
The correct answer would be A. First of all, it has the word modern in it. Second of all, the birds we know today would have certainly been a lot different 66+ mya.