"Which is considered by experts/scientists as the most impressive type of cooperation between monkeys? It does not concern survival,motherhood,feeding, scratching, adoption, danger. It probably happens on their move and could be the solution or dealing with a problem."
I did a search on "monkey cooperation" and found a few sites whewre people were asking EXACTLY the same question, so it seems this is part of a quiz or maybe an exam.
There is a study of capuchin monkeys that seems to demonstrate they have a developed sense of fairness. Google 'capuchin monkeys sense of fairness' for a lot of links.