The correct answer is option 3: Hiuen Tsang. Hiuen Tsang, also known as Xuanzang, was a Chinese Buddhist monk and scholar who travelled extensively in India during the 7th century AD. He is famous for his detailed accounts of the kingdoms he visited, including Harshavardhana`s kingdom in northern India and Kumar Bhaskar Varma`s kingdom in present-day Assam, India.

Option 1: I-tsing was another Chinese Buddhist monk and scholar who also visited India, but he did not specifically mention visiting the kingdoms of Harshavardhana and Kumar Bhaskar Varma.

Option 2: Fa-Hien, also known as Faxian, was an earlier Chinese traveller who visited India in the 5th century AD. While he wrote about his journey and the places he visited, there is no mention of him visiting the kingdoms of Harshavardhana and Kumar Bhaskar Varma.

Option 4: Sun Shuyun is not a known Chinese traveller who visited India during this time period.

Therefore, the correct answer is option 3: Hiuen Tsang.