The correct answer is option 3 - Megasthenes. Megasthenes was the ambassador sent to the court of Chandragupta Maurya by the Greek ruler of West Asia, Selucus Nicator. Megasthenes was a Greek historian and diplomat who visited the Mauryan Empire during the reign of Chandragupta. He wrote a book called "Indica" which provides valuable insights into the political, social, and economic conditions of ancient India. In his book, Megasthenes gives a detailed account of the administration, military, society, and customs of the Mauryan Empire. His writings are considered important historical sources for understanding the Mauryan period.