Which one of the following periods is longest in the human history?

The Palaeolithic (Old Stone Age) period is the longest in human history, spanning roughly from 2.5 million years ago to about 10,000 BCE. It encompasses the vast majority of human existence. The Mesolithic (Middle Stone Age) lasted only a few thousand years (around 10,000-8,000 BCE), the Neolithic (New Stone Age) about 5,000 years, and the Megalithic period even shorter.