Correct option is B
Option (b) is the correct order of the given sentences.
Explanation of the correct order DACB:
D: "Nepal, long under the rule of hereditary prime ministers..." — This introduces the historical background and sets the context, making it the natural opening sentence.
A: "The country gained admission to the United Nations in 1955." — Follows the restoration of the monarchy and shows Nepal opening up to the world.
C: "In 1991 the kingdom established a multiparty parliamentary system." — This indicates a gradual political progression.
B: "In 2008, however, after a decade long period of violence..." — This comes at the end, showing the culmination of the democratic transition.