The correct option is
A Precambrian
Trilobites are a fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the stage of the Precambrian period around 521 million years ago, and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian period, all trilobite orders except Proetida died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago.
So, the correct answer is 'Precambrian'