Answer:
D. a reduced conflict and increased harmony between the two families
Explanation:
Cooperative behavior: The term "cooperative behavior" is described as an interaction between two or more than two organizations or individuals that are being directed towards a specific goal which is considered as "mutually beneficial". In other words, it is described as an instance or act related to acting or working with each other for a mutual benefit or purpose.
In the question above, the given families who had been enemies for specific years are now working together to save an overflowing river which leads to reduce conflict and increase harmony between them.