The correct answer is option D, Chandragupta Maurya.

Explanation:
Megasthenes, a Greek historian, visited India during the reign of Chandragupta Maurya in the 4th century B.C. Chandragupta Maurya was the founder of the Mauryan Empire, which was one of the largest and most powerful empires in ancient India.

Chandragupta Maurya:
Chandragupta Maurya was born in the 4th century B.C. and he established the Mauryan Empire in 322 B.C. after overthrowing the Nanda dynasty. He was the first ruler of the Mauryan dynasty and ruled from 322 B.C. to 298 B.C. Chandragupta Maurya's reign was known for its political and administrative achievements, and he was able to expand his territory through military conquests.

Megasthenes' Visit to India:
Megasthenes' visit to India during Chandragupta Maurya's reign is important because it provides valuable information about ancient India, its culture, society, and administration. Megasthenes wrote a book called "Indica" based on his observations and experiences in India. Although the original text is lost, fragments and references from other ancient writers have provided insights into Megasthenes' work.

Megasthenes' "Indica":
Megasthenes' "Indica" is considered a significant source of information about ancient India. It described various aspects of Indian society, including its political system, economy, religion, and customs. Megasthenes provided detailed accounts of the Mauryan administration, describing the hierarchical structure and the efficient bureaucracy of the empire. He also wrote about the capital city of Pataliputra (modern-day Patna) and the grandeur of the Mauryan court.

Conclusion:
In conclusion, Megasthenes, a Greek historian, visited India during the reign of Chandragupta Maurya in the 4th century B.C. His book "Indica" provides valuable insights into ancient Indian society and the Mauryan Empire. Chandragupta Maurya's reign was significant in the history of India, and Megasthenes' visit and writings have contributed to our understanding of that period.