In 2013, the World Water Week will be held under the theme Water ________________-Building Partnerships. (COOPERATE)

The correct answer is "COOPERATION."

In this context, the sentence requires a noun to complete the theme of "Water ________________-Building Partnerships." The word "COOPERATE" is a verb, which means to work together or collaborate. However, to fit grammatically into the sentence, we need the noun form, which is "COOPERATION."

"Cooperation" refers to the process of working together to the same end, which aligns perfectly with the theme regarding building partnerships in the context of water management. Thus, the completed sentence reads: "In 2013, the World Water Week will be held under the theme Water COOPERATION - Building Partnerships."