The correct answer is: A. Rohtas Plateau.
The Rohtas Plateau is a geologically older part of Bihar. It is a plateau that is located in the western part of the state. The plateau is made up of Precambrian rocks, which are some of the oldest rocks in the world. The plateau is also home to a number of important archaeological sites, including the ruins of the ancient city of Rajgir.
The North-Western Hills are a geologically younger part of Bihar. They are a range of hills that are located in the northwestern part of the state. The hills are made up of rocks that are from the Mesozoic era, which is a geological period that lasted from about 252 to 66 million years ago. The hills are home to a number of important wildlife sanctuaries, including the Sanjay Gandhi National Park.
The Kharagpur Hills are a geologically younger part of Bihar. They are a range of hills that are located in the southeastern part of the state. The hills are made up of rocks that are from the Cenozoic era, which is a geological period that began about 66 million years ago and continues to the present day. The hills are home to a number of important tea plantations.
The North Ganga Plains are a geologically younger part of Bihar. They are a plain that is located in the northern part of the state. The plain is made up of alluvial deposits, which are sediments that have been deposited by rivers. The plain is home to a number of important agricultural areas.
Therefore, the Rohtas Plateau is geologically older than the other parts of Bihar.