Marco Polo's primary contribution to the exploration of the world was that he wrote a book about Asia. His work, "The Travels of Marco Polo," detailed his journey through Asia, particularly his experiences in China, and provided Europeans with a wealth of information about the cultures, geography, and commerce of the East. This book significantly influenced later explorers and sparked interest in exploration and trade with Asia, including the Silk Road.
The other options you mentioned do not accurately represent his contributions:
- He did not sail a caravel to India; that type of ship is associated with later explorers such as Vasco da Gama.
- He did not focus on converting Asians to Christianity; his accounts were more about the cultures and customs he encountered.
- Marco Polo did not invent the astrolabe; this navigational instrument had been developed centuries earlier.
Therefore, writing about Asia is his most notable contribution to exploration.