Which two provinces have never merged since their establishment?

These are two regions that have never been merged or separated, and have remained the same since the feudal period until now.

Correct

In 1976, Vietnam had 38 provinces and cities, including 35 provinces and 3 centrally-run cities. This was the year with the smallest number of provinces and cities in Vietnam in the past half century.
At that time, the North had 13 provinces and 2 cities including: Bac Thai, Cao Lang, Ha Nam Ninh, Ha Bac, Ha Son Binh, Ha Tuyen, Hai Hung, Hoang Lien Son, Lai Chau, Quang Ninh, Son La, Thai Binh , Vinh Phu. The two cities were Hanoi and Hai Phong.
Central region: 10 provinces: Thanh Hoa, Nghe Tinh, Binh Tri Thien, Quang Nam - Da Nang, Nghia Binh, Phu Khanh, Thuan Hai, Gia Lai - Kon Tum, Dak Lak, Lam Dong.
Southern 12 provinces: Song Be, Tay Ninh, Dong Nai, Long An, Dong Thap, An Giang, Tien Giang , Hau Giang, Kien Giang, Ben Tre, Cuu Long, Minh Hai and Ho Chi Minh City.

Source: https://vtcnews.vn/hai-tinh-nao-chua-tung-sap-nhap-tu-khi-thanh-lap-ar930922.html