Who was Megasthenes?

The correct answer is Option (1) → A Greek ambassador to the court of Chandragupta Maurya.

Megasthenes was sent by the Greek ruler Seleucus I Nicator as his ambassador to the court of Chandragupta Maurya at Pataliputra, around 300 BCE, after the two kings made peace. He described the Mauryan capital, its administration and Indian society in his book Indica, which is lost and survives only in quotations by later Greek and Roman writers.