A military conflict was avoided because of a diplomatic effort by the much respected ambassador. Diplomatic most nearly means:
"Diplomatic" can mean tactful. Using tact in your relationships with others means that you speak and behave in a way that doesn't offend other persons with a different point of view. That "diplomacy" is necessary when an ambassador talks with a world leader. But diplomacy doesn't mean being submissive, so don't be distracted by that wrong answer.