The correct answer is: b) Ancient mountain ranges.
Monadonocks are isolated hills that rise above the surrounding landscape. They are formed when the surrounding land is eroded away, leaving the monadnocks standing as remnants of a once-larger mountain range.
The southern parts of Bihar are home to a number of monadnocks, which are thought to be remnants of the ancient Vindhyan mountain range. The Vindhyan mountains were once a major mountain range that stretched across central India, but they have been eroded away over time. The monadnocks that are found in Bihar are the last remaining parts of this ancient mountain range.
The other options are incorrect because they do not explain the formation of monadnocks. Recent volcanic activity does not typically result in the formation of hills, and glacial deposits and river erosion are processes that can remove hills, not create them.