The correct answer is: D. 1946 AD
The Himalayan Hill States Regional Council was formed in 1946 AD. It was a regional council of the princely states of the Himalayas. The council was formed to promote cooperation and coordination among the princely states, and to represent their interests to the British government. The council was dissolved in 1947, after the independence of India.
Option A: 1940 AD is incorrect. The Himalayan Hill States Regional Council was not formed in 1940 AD.
Option B: 1942 AD is incorrect. The Himalayan Hill States Regional Council was not formed in 1942 AD.
Option C: 1945 AD is incorrect. The Himalayan Hill States Regional Council was not formed in 1945 AD.
Option D: 1946 AD is correct. The Himalayan Hill States Regional Council was formed in 1946 AD.