The correct answer is option 1: Megasthenes was a Greek ambassador to the court of Chandragupta Maurya. Megasthenes was a Greek historian and diplomat who lived in the 3rd century BC. He was sent by the Greek ruler Seleucus I Nicator as an ambassador to the court of the Maurya Empire`s ruler, Chandragupta Maurya. His purpose was to establish diplomatic relations and gather information about the Mauryan Empire.

During his time in India, Megasthenes wrote a book called "Indica" which provided valuable insights into the customs, culture, and political systems of ancient India. Although parts of his work have been lost, the surviving fragments give us important knowledge about the Mauryan Empire and its capital, Pataliputra (modern-day Patna, India). Megasthenes` account of India is considered one of the earliest and most detailed descriptions of the country by an outsider.

Therefore, Megasthenes was not a trader or a Chinese pilgrim, as mentioned in options 2, 3, and 4.