This is the 4636th most frequent Russian word.
дипломатического
"Diplomatic"
In this sentence, 'дипломатического' (diplomatic) is used as an adjective describing the nature of the approach needed.
Дипломатического подхода требуется для решения конфликта.
A diplomatic approach is required to resolve the conflict.
Here, 'дипломатического' (diplomatic) modifies 'участия' (participation), specifying the context of the involvement in international discussions.
Он изучает историю дипломатического участия страны в международных переговорах.
He studies the history of the country's diplomatic participation in international negotiations.
'Дипломатического' (diplomatic) in this case characterizes 'уровня' (level), indicating the formal level of dialogue.