The correct answer is: A. Megasthenes
Megasthenes was a Greek ambassador to the court of Chandragupta Maurya in the 4th century BCE. He wrote a book called Indica, which is a valuable source of information about the Mauryan Empire. In Indica, Megasthenes states that the Mauryan administration in Patliputra was run by 30 citizens who were divided into 6 committees.
The other options are incorrect. Kautilya was a Brahmin scholar who served as the chief advisor to Chandragupta Maurya. He is the author of the Arthashastra, a treatise on statecraft. Nagarjuna was a Buddhist philosopher who lived in the 2nd century CE. Vasubandhu was a Buddhist monk who lived in the 5th century CE.
Megasthenes’ account of the Mauryan administration is generally considered to be reliable. However, it is important to note that he was writing from the perspective of an outsider, and his account may not be entirely accurate.