The correct answer is Megasthenes.
Megasthenes was a Greek ambassador to the court of Chandragupta Maurya in the 4th century BCE. He wrote a detailed account of India, which is one of the most important sources of information about the Mauryan Empire.
Fa Hien was a Chinese Buddhist monk who visited India in the 5th century CE. He wrote a book about his travels, which is an important source of information about Indian Buddhism.
Xuanzang was a Chinese Buddhist monk who visited India in the 7th century CE. He wrote a book about his travels, which is an important source of information about Indian Buddhism and the history of the Tang dynasty.
Marco Polo was a Venetian merchant who visited India in the 13th century CE. He wrote a book about his travels, which is an important source of information about India during the Mongol Empire.
Megasthenes was the only one of these travelers who visited Pataliputra during the Mauryan Period.