The correct answer is (b), Dehradun and Champawat.
Dehradun is the most populous district in Uttarakhand, with a population of 1,361,704 in 2011. It is the state capital and is located in the Garhwal region. The city is a major tourist destination and is home to several educational institutions, including the University of Dehradun.
Champawat is the least populous district in Uttarakhand, with a population of 349,034 in 2011. It is located in the Kumaon region and is known for its natural beauty. The district is home to several wildlife sanctuaries, including the Rajaji National Park.
The other options are incorrect. Haridwar is the second most populous district in Uttarakhand, with a population of 1,059,698 in 2011. Udhamsingh Nagar is the third most populous district, with a population of 1,032,736 in 2011. Nainital is the fourth most populous district, with a population of 960,789 in 2011. Rudraprayag is the fifth most populous district, with a population of 548,195 in 2011. Bageshwar is the sixth most populous district, with a population of 432,390 in 2011. Chamoli is the seventh most populous district, with a population of 391,724 in 2011.