The correct answer is option 3, Megasthenes. Megasthenes was the Greek ambassador sent to Chandragupta Maurya`s court. He was a historian and diplomat who visited the court of Chandragupta Maurya, the founder of the Maurya Empire, during the 4th century BCE. Megasthenes wrote a detailed account of his observations and experiences in India, known as the "Indika," which provides valuable information about the political, social, and cultural aspects of ancient India. His writings were one of the earliest and most comprehensive sources of information about ancient India for the Western world. Therefore, Megasthenes is the correct answer.