Peace keeping for the United Nations is, at best of times and arduous task.

Answer: Option C

Laborious : requiring considerable time and effort.
Inconvenient : causing trouble, difficulties, or discomfort.
Difficult : needing much effort or skill to accomplish, deal with, or understand.
Delicate : very fine in texture or structure; of intricate workmanship or quality.
Arduous : involving or requiring strenuous effort; difficult and tiring.

Synonym of Arduous is Difficult