D had recently visited Khardung La, Nubra Valley, and several Buddhist monasteries. To determine which state or union territory (UT) she had visited, let`s analyze each option:

Option 1: Himachal Pradesh - This state is known for its scenic beauty and popular tourist destinations like Shimla and Manali. However, Khardung La, Nubra Valley, and Buddhist monasteries are not located in Himachal Pradesh. Thus, option 1 is incorrect.

Option 2: Uttarakhand - Uttarakhand is famous for its hill stations like Mussoorie and Nainital. However, Khardung La, Nubra Valley, and Buddhist monasteries are not part of this state. Hence, option 2 is incorrect.

Option 3: Sikkim - Sikkim is known for its breathtaking landscapes and is home to many Buddhist monasteries. Although it shares borders with Nepal, Bhutan, and Tibet, it does not include Khardung La or Nubra Valley. Therefore, option 3 is also incorrect.

Option 4: Ladakh - Ladakh, a union territory in India, is known for its high altitude passes, stunning landscapes, and Buddhist monasteries