The correct answer is (a) Uttarkashi.
Uttarkashi is the largest district in Uttarakhand, India. It is located in the Garhwal region of the state. The district has a population of over 500,000 people and a geographical area of over 10,000 square kilometers. The district is home to several important Hindu pilgrimage sites, including Kedarnath, Badrinath, and Gangotri.
Chamoli is the second largest district in Uttarakhand. It is located in the Garhwal region of the state. The district has a population of over 400,000 people and a geographical area of over 9,000 square kilometers. The district is home to several important Hindu pilgrimage sites, including Yamunotri and Hemkund Sahib.
Pithoragarh is the third largest district in Uttarakhand. It is located in the Kumaon region of the state. The district has a population of over 300,000 people and a geographical area of over 8,000 square kilometers. The district is home to several important Hindu pilgrimage sites, including Nanda Devi and Almora.
Dehradun is the fourth largest district in Uttarakhand. It is located in the Garhwal region of the state. The district has a population of over 1 million people and a geographical area of over 3,000 square kilometers. The district is the capital of Uttarakhand and is home to several important government offices and educational institutions.