A Greek ambassador Megasthenes came to the court of which Indian ruler?

Chandragupta Maurya

The correct answer is Option (1) → Chandragupta Maurya

Historians have used a variety of sources to reconstruct the history of the Mauryan Empire. These include archaeological finds, especially sculpture. Also valuable are contemporary works, such as the account of Megasthenes (a Greek ambassador to the court of Chandragupta Maurya), which survives in fragments. Another source that is often used is the Arthashastra, parts of which were probably composed by Kautilya or Chanakya, traditionally believed to be the minister of Chandragupta. Besides, the Mauryas are mentioned in later Buddhist, Jaina and Puranic literature, as well as in Sanskrit literary works. While these are useful, the inscriptions of Asoka (c. 272/268-231 BCE) on rocks and pillars are often regarded as amongst the most valuable sources.