The correct answer is: A. Thiksey
Thiksey Monastery is the oldest and the largest Buddhist monastery in the Nubra Valley of Ladakh. It is located on a hilltop overlooking the village of Thiksey, about 40 kilometers from Leh. The monastery was founded in the 15th century by Rinchen Namgyal, a disciple of Tsongkhapa, the founder of the Gelugpa school of Tibetan Buddhism. The monastery is home to over 500 monks and is a major center of learning and pilgrimage.
Diskit Monastery is the second largest monastery in the Nubra Valley. It is located in the village of Diskit, about 20 kilometers from Leh. The monastery was founded in the 14th century by Rinchen Sangpo, a disciple of Tsongkhapa. The monastery is home to over 200 monks and is a major center of learning and pilgrimage.
Takthok Monastery is a small monastery located in the village of Takthok, about 10 kilometers from Leh. The monastery was founded in the 14th century by Rinchen Sangpo, a disciple of Tsongkhapa. The monastery is home to about 50 monks and is a major center of learning and pilgrimage.
None of these options is the oldest and the largest Buddhist monastery in the Nubra Valley of Ladakh. The oldest and the largest Buddhist monastery in the Nubra Valley is Thiksey Monastery.