Which era is directly preceding the Mesozoic Era?

The era that directly precedes the Mesozoic Era is the Paleozoic Era. This era, which lasted from approximately 541 to 251 million years ago, is characterized by a significant increase in biodiversity, the development of early life forms including marine organisms, and the eventual rise of vertebrates. The transition from the Paleozoic to the Mesozoic was marked by the Permian-Triassic extinction event, one of the most profound extinction events in Earth’s history, which paved the way for the dominance of reptiles and the eventual rise of dinosaurs during the Mesozoic.

The other eras listed, such as the Cenozoic, come after the Mesozoic and are notable for different geological and biological developments but do not precede it. The Proterozoic and Archean Eras represent much earlier periods in Earth's history, with the Proterozoic dating back to around 2.5 billion years ago and the Archean to over 4 billion years ago. These eras primarily focus on the formation of the Earth and the emergence of simple life forms. Consequently, when considering the sequence of geological periods, it is clear that the Paleozoic is the correct answer.