The correct answer is (a) Pithoragarh.
Pithoragarh is the largest district in Uttarakhand, with a geographical area of 9,062 square kilometers. It is located in the Kumaon region of the state, and is bordered by Nepal to the east, China to the north, and the districts of Chamoli, Bageshwar, Almora, and Nainital to the south, west, and northwest, respectively. The district is home to a number of important tourist destinations, including the Nanda Devi National Park, the Valley of Flowers National Park, and the Pithoragarh Fort.
Uttarkashi is the second largest district in Uttarakhand, with a geographical area of 8,586 square kilometers. It is located in the Garhwal region of the state, and is bordered by the districts of Chamoli, Rudraprayag, Tehri Garhwal, Dehradun, and Haridwar to the south, west, north, and east, respectively. The district is home to the Kedarnath Temple, one of the most important Hindu pilgrimage sites in India.
Pauri Garhwal is the third largest district in Uttarakhand, with a geographical area of 8,060 square kilometers. It is located in the Garhwal region of the state, and is bordered by the districts of Tehri Garhwal, Dehradun, Haridwar, and Udham Singh Nagar to the south, west, north, and east, respectively. The district is home to the Auli ski resort, one of the most popular tourist destinations in Uttarakhand.
Tehri Garhwal is the fourth largest district in Uttarakhand, with a geographical area of 7,409 square kilometers. It is located in the Garhwal region of the state, and is bordered by the districts of Pauri Garhwal, Chamoli, Rudraprayag, Dehradun, and Haridwar to the south, west, north, and east, respectively. The district is home to the Tehri Dam, the largest dam in India.