Megasthenes was a Greek ambassador in the court of ______.

Correct option is D

The correct answer is (d) Chandragupta Maurya.
Megasthenes was a Greek historian and diplomat who served as an ambassador of Seleucus I Nicator to the court of Chandragupta Maurya, the founder of the Maurya Empire.
Megasthenes wrote a book about India called "Indica", which is a valuable source of information about ancient India. The book is now lost, but it has been preserved in fragments quoted by later authors.
Other Greek envoys to the Indian court are known after Megasthenes: Deimachus as ambassador to Bindusara, and Dionysius, as ambassador to Ashoka.