The correct answer is A. Pre-Cambrian.
The Pre-Cambrian is the first geological eon in the history of Earth. It lasted from the formation of Earth about 4.54 billion years ago to the beginning of the Cambrian period about 541 million years ago. The Pre-Cambrian is the longest eon in Earth’s history, spanning over 4 billion years.
The Southern plateau of Bihar is a part of the Deccan Plateau, which is a large plateau in central and western India. The Deccan Plateau is made up of Precambrian rocks, which are the oldest rocks on Earth. The Precambrian rocks in the Deccan Plateau were formed by the accretion of volcanic and sedimentary material.
The Oligocene is the third epoch of the Paleogene period and lasted from 33.9 to 23.03 million years ago. The Oligocene is characterized by the rise of the first modern mammals, the decline of the dinosaurs, and the beginning of the formation of the Himalayas.
The Miocene is the fourth epoch of the Neogene period and lasted from 23.03 to 5.33 million years ago. The Miocene is characterized by the rise of the first modern humans, the expansion of the grasslands, and the beginning of the glaciation of the Northern Hemisphere.
The Pleistocene is the first epoch of the Quaternary period and lasted from 2.588 million to 11,700 years ago. The Pleistocene is characterized by the repeated glaciation of the Northern Hemisphere and the rise of modern humans.
Therefore, the correct answer is A. Pre-Cambrian.