The correct answer is (a) 12th Feb. 1816 A.D.
The princely State of Tehri was formed in Tehri on 12th Feb. 1816 A.D. by Maharaja Pratap Shah. The state was founded after the Gorkha War of 1814-1816, when the British East India Company defeated the Gorkha Kingdom and annexed the Garhwal region. The British then created the princely State of Tehri as a buffer state between the British-controlled territories and the independent Kingdom of Nepal.
The capital of the princely State of Tehri was initially located in Dehradun, but it was moved to Tehri in 1816. The city of Tehri was founded by Maharaja Pratap Shah and was named after him. Tehri was the capital of the princely State of Tehri until it was merged with the Indian Union in 1949.
The other options are incorrect because they are not the dates on which the princely State of Tehri was formed.