Explanation:

- King Shashanka was the ruler of the Gauda Kingdom.

- Option 1: Kanyakubja - This was another name for the Kanauj kingdom, but it was not ruled by Shashanka. Kanauj was significant during Harshavardhana’s rule.

- Option 2: Junagarh - Located in modern-day Gujarat, this was not related to King Shashanka's reign or his conflicts.

- Option 3: Magadha - This historical region was a major kingdom in ancient India but was not ruled by Shashanka during Harshavardhana's period.

- Option 4: Gauda - This is the correct option. Shashanka was the ruler of Gauda, now part of West Bengal and Bangladesh. Harshvardhana declared war against this kingdom.