**The Periplus of the Erythrean Sea: An Overview**

The Periplus of the Erythrean Sea is an ancient Greek travelogue that provides valuable insights into the maritime trade network between the Roman Empire and the Indian subcontinent during the 1st century CE. It was written by an anonymous author, commonly referred to as the "Periplus author," who was likely a Greek merchant or traveler.

**Content of the Periplus of the Erythrean Sea**

The book focuses primarily on the trade routes, ports, and products exchanged between the Roman Empire and the Indian subcontinent. It describes the navigational directions, distances, and locations of various ports along the Red Sea, the Arabian Sea, and the Indian Ocean. Consequently, the main subject matter of the book revolves around the Indian exports to the Roman world, making option C the correct answer.

**Indian Exports to the Roman World**

The Periplus of the Erythrean Sea provides detailed information about the various products that were exported from India to the Roman Empire. Some of the notable Indian exports mentioned in the book include:

1. Spices: The book extensively discusses the trade of spices, such as pepper, cinnamon, and ginger, which were highly sought after by the Romans. It describes the regions in India where these spices were produced and the different types available.

2. Textiles: The Periplus also highlights the trade in Indian textiles, particularly cotton and silk fabrics. It mentions the cities and ports where these textiles were manufactured and exported, such as Musiris (modern-day Kodungallur) and Muziris (modern-day Pattanam) in present-day Kerala.

3. Precious Stones: Another significant export from India to the Roman world was precious stones, including diamonds, pearls, and emeralds. The book mentions the different regions in India known for their gemstone production and trading hubs.

4. Indigo: The Periplus provides insights into the trade of indigo, a plant-based dye used for coloring textiles. It mentions the coastal regions of India where indigo was cultivated and exported.

Apart from these products, the Periplus also touches upon other commodities like ivory, timber, tortoise shells, and exotic animals that were part of the Indian exports to the Roman world.

**Conclusion**

In conclusion, the Periplus of the Erythrean Sea primarily deals with the Indian exports to the Roman world. It provides valuable information about the trade routes, ports, and the various products exchanged between the two regions during the 1st century CE. The book's content is a valuable historical resource for understanding the maritime trade network of the ancient world and the economic relations between the Roman Empire and the Indian subcontinent.