The correct answer is (a) Mussoorie.
Sir George Everest established his observatory in Mussoorie in 1830. He was a British Surveyor General of India from 1830 to 1843. He is best known for his work on the Great Trigonometric Survey of India, which was a project to map the entire Indian subcontinent. The highest mountain in the world, Mount Everest, was named after him in 1865.
Mussoorie is a hill station in the Indian state of Uttarakhand. It is located in the Garhwal Himalayas, at an altitude of 2,205 meters (7,234 feet). The town is a popular tourist destination, known for its scenic views, pleasant climate, and colonial architecture.
The other options are incorrect.
(b) Nainital is a town and a lake in the Kumaon region of Uttarakhand, India. It is a popular tourist destination, known for its scenic beauty, boating, and other water sports.
(c) Devsthal is a village in the Tehri Garhwal district of Uttarakhand, India. It is located at an altitude of 2,400 meters (7,874 feet). The village is a popular destination for trekking and mountaineering.
(d) Abbot Mount is a mountain in the Garhwal Himalayas, India. It is located in the Chamoli district, at an altitude of 6,852 meters (22,480 feet). The mountain is a popular destination for mountaineering.