The correct answer is A. Gulab Singh.
Mubarak Mandi Palace was built by Gulab Singh, the first Maharaja of Jammu and Kashmir, in the 19th century. It is a complex of over 200 buildings, including palaces, temples, mosques, and gardens. The palace was built in the Mughal style of architecture and is considered to be one of the finest examples of this style in India. It is now a museum and a popular tourist destination.
B. Dhruv Dev was a Hindu king who ruled the Chamba Kingdom in the 11th century. He is credited with building the Chamba Fort, which is one of the most important historical monuments in the state of Himachal Pradesh.
C. Hari Singh was the last Maharaja of Jammu and Kashmir. He ruled from 1925 to 1950. He is best known for his decision to accede to India in 1947, which led to the partition of Jammu and Kashmir and the beginning of the Kashmir conflict.
D. None of these is the correct answer.