The correct answer is (b) Dehradun.
Dehradun is the most populous district in Uttarakhand, with a population of 1,011,708 as per the 2011 Census. It is the state capital and the largest city in the state. Dehradun is located in the Doon Valley, which is a popular tourist destination. The city is also home to a number of educational institutions, including the Indian Institute of Technology Roorkee and the University of Dehradun.
Almora is the second most populous district in Uttarakhand, with a population of 563,781 as per the 2011 Census. It is located in the Kumaon region of the state. Almora is a popular tourist destination, known for its scenic beauty and its historical monuments.
Haridwar is the third most populous district in Uttarakhand, with a population of 548,132 as per the 2011 Census. It is located in the Garhwal region of the state. Haridwar is a holy city for Hindus, and is a popular pilgrimage destination.
Udham Singh Nagar is the fourth most populous district in Uttarakhand, with a population of 474,314 as per the 2011 Census. It is located in the Terai region of the state. Udham Singh Nagar is a major agricultural district, and is also home to a number of industrial units.