The correct answer is: B. Fa-Hien
Fa-Hien was a Chinese Buddhist monk who traveled to India in search of Buddhist scriptures. He is believed to have visited Bihar in the 5th century CE. He wrote a book about his travels, which is an important source of information about India at that time.
Megasthenes was a Greek ambassador to India in the 4th century BCE. He wrote a book about India, which is also an important source of information about India at that time. However, there is no evidence that he visited Bihar.
Hiuen Tsang was a Chinese Buddhist monk who traveled to India in the 7th century CE. He is also believed to have visited Bihar. He wrote a book about his travels, which is another important source of information about India at that time.
I-Tsing was a Chinese Buddhist monk who traveled to India in the 7th century CE. He is also believed to have visited Bihar. He wrote a book about his travels, which is another important source of information about India at that time.
However, Fa-Hien is the only one of these four travelers who is definitely known to have visited Bihar.