The correct answer is: A. six
The Paleozoic Era is the first of three geologic eras of the Phanerozoic eon, spanning 541 to 252 million years ago. It is the longest of the Phanerozoic eras, and is subdivided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
The Cambrian Period was a time of great evolutionary change, when most of the major animal groups first appeared. The Ordovician Period was a time of further diversification of life, and the Silurian Period saw the rise of the first land plants. The Devonian Period was a time of great change for life on land, as the first forests appeared and the first amphibians evolved. The Carboniferous Period was a time of great coal formation, as the first forests grew and died. The Permian Period was a time of great extinction, when most of the marine life on Earth died out.
The Paleozoic Era was a time of great change and evolution on Earth. It saw the rise of the first animals, plants, and forests, and the development of the first land ecosystems. It was also a time of great extinction, when most of the marine life on Earth died out.