Explanation:

- Assam has a distinct administrative history in British India.

- Option 1: It was a part of the North-East Frontier Agency. - Incorrect, as this agency was formed after India’s independence.

- Option 2: It was made a province in 1865. - Assam became a Chief Commissioner's province after 1874, not in 1865.

- Option 3: It was separated from Bengal in 1874, and along with Sylhet made into a Chief Commissioner's province. - This accurately describes Assam's status at that time.

- Option 4: It was a Princely State ruled by Ahom kings. - The Ahom rule ended long before British administration.