The correct answer is (b), Dehradun and Champawat.
Dehradun is the most populous district in Uttarakhand, with a population of 1,361,914 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 316,242 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 Jim Corbett National Park.
The other options are incorrect. Haridwar is the second most populous district in Uttarakhand, with a population of 1,191,720 in 2011. It is a holy city for Hindus and is located on the banks of the Ganges River. Udhamsingh Nagar is the third most populous district in Uttarakhand, with a population of 1,009,225 in 2011. It is located in the Kumaon region and is home to several industrial towns. Bageshwar is the fourth most populous district in Uttarakhand, with a population of 447,103 in 2011. It is located in the Kumaon region and is known for its natural beauty. Nainital is the fifth most populous district in Uttarakhand, with a population of 446,592 in 2011. It is a popular tourist destination and is located in the Kumaon region. Chamoli is the sixth most populous district in Uttarakhand, with a population of 356,499 in 2011. It is located in the Garhwal region and is home to several glaciers, including the Nanda Devi Glacier.