Which one is part of Phanerozoic era?

The correct answer is Paleozoic. The Phanerozoic Eon is the current and most recent of the four geologic eons, spanning the last 541 million years. It is divided into three major eras: the Paleozoic, the Mesozoic, and the Cenozoic.

Geological Time and the Phanerozoic Eon

• Phanerozoic Subdivisions: The Phanerozoic means "visible life" and is characterized by abundant fossil evidence. Its eras are:
  • Paleozoic Era (541–252 million years ago): Age of invertebrates, fish, amphibians, and early reptiles. Ends with the Permian‑Triassic mass extinction.
  • Mesozoic Era (252–66 million years ago): Age of reptiles, including dinosaurs.
  • Cenozoic Era (66 million years ago–present): Age of mammals.
• Precambrian Supereon: The Archean, Hadean, and general Precambrian all belong to the time before the Phanerozoic. They represent the vast bulk of Earth's history (over 4 billion years) but lack abundant visible fossils.
• Why Only Paleozoic Fits: The question asks which one is part of the Phanerozoic era. The Paleozoic is indeed the first era of the Phanerozoic. The other options (Archean, Hadean, Precambrian) are all pre‑Phanerozoic divisions.
• Significance: This division marks the explosive diversification of life known as the Cambrian explosion, which occurred at the beginning of the Paleozoic Era.

Thus, the Paleozoic is the era that belongs to the Phanerozoic Eon.