The correct answer is option 'A', Paleolithic.

Explanation:

The Stone Age is a prehistoric period characterized by the use of stone tools. It is divided into three main periods: Paleolithic, Mesolithic, and Neolithic. Among these, the Paleolithic period is the longest in human history.

1. Paleolithic period:
- Also known as the Old Stone Age, it lasted from around 2.5 million years ago to about 10,000 years ago.
- It is further divided into three subdivisions: Lower Paleolithic, Middle Paleolithic, and Upper Paleolithic.
- During this period, early humans were hunter-gatherers, relying on hunting animals and gathering wild plants for survival.
- Stone tools were used for various purposes, such as hunting, cutting, and scraping.
- Art and symbolism also emerged during the Upper Paleolithic, as evidenced by cave paintings and figurines.

2. Mesolithic period:
- Also known as the Middle Stone Age, it followed the Paleolithic period and lasted from about 10,000 to 5,000 years ago.
- It was a transitional period between the Paleolithic and Neolithic periods.
- During this period, humans began to domesticate animals and engage in more settled lifestyles.
- Stone tools became more refined, and fishing and gathering activities increased.
- The Mesolithic period witnessed the development of more complex societies and the beginning of pottery production.

3. Neolithic period:
- Also known as the New Stone Age, it lasted from about 5,000 to 2,000 years ago.
- It marked a significant shift in human history as humans started practicing agriculture and domesticating plants and animals.
- Settlements became more permanent, and the development of agriculture led to the rise of complex societies.
- Stone tools were still used, but polished stone tools and pottery became more common during this period.

4. Megalithic period:
- The Megalithic period refers to a specific form of architectural style characterized by the use of large stones to construct monuments, tombs, and other structures.
- It is not a distinct period within the Stone Age, but rather a cultural phenomenon that occurred during different periods in different regions.
- Megalithic structures can be found in various parts of the world, including Europe, Africa, and Asia.

In conclusion, the Paleolithic period is the longest Stone Age period in human history, spanning from around 2.5 million years ago to about 10,000 years ago.