The correct answer is (a) Uttarkashi.
Uttarkashi is the largest district in Uttarakhand, with a geographical area of 13,826 square kilometers. It is located in the Garhwal region of the state and is bordered by Himachal Pradesh to the north, Chamoli district to the east, Pithoragarh district to the south, and Dehradun district to the west. The district is home to the Kedarnath and Badrinath temples, two of the most important Hindu pilgrimage sites in India.
Chamoli is the second largest district in Uttarakhand, with a geographical area of 12,596 square kilometers. It is located in the Garhwal region of the state and is bordered by Uttarkashi district to the north, Pithoragarh district to the east, Kumaon district to the south, and Dehradun district to the west. The district is home to the Nanda Devi National Park, which is one of the most popular tourist destinations in Uttarakhand.
Pithoragarh is the third largest district in Uttarakhand, with a geographical area of 9,072 square kilometers. It is located in the Kumaon region of the state and is bordered by China to the north, Nepal to the east, Chamoli district to the south, and Almora district to the west. The district is home to the Nanda Devi Biosphere Reserve, which is a UNESCO World Heritage Site.
Dehradun is the fourth largest district in Uttarakhand, with a geographical area of 2,405 square kilometers. It is located in the Garhwal region of the state and is bordered by Haridwar district to the north, Pauri Garhwal district to the east, Tehri Garhwal district to the south, and Uttarkashi district to the west. The district is the capital of Uttarakhand and is home to the Doon Valley, which is a popular tourist destination.