The correct answer is D. Keylong.
Keylong is a town in Lahaul and Spiti district in the Indian state of Himachal Pradesh. It is the administrative headquarters of the district. Keylong is located at an altitude of 3,300 meters (10,800 feet) above sea level. The town is surrounded by mountains and is a popular tourist destination.
Keylong is called the Gateway of Lahaul because it is the first town that visitors reach when they travel to Lahaul from the rest of India. The town is located at the entrance to the Lahaul Valley, which is a beautiful and remote valley in the Himalayas.
The other options are incorrect because they are not located in Lahaul. Khoskar is a village in the Kullu district of Himachal Pradesh. Markula is a village in the Chamba district of Himachal Pradesh. Tandi is a town in the Lahaul and Spiti district of Himachal Pradesh.