All of the following theory of international relation has pessimistic attitude toward international relation EXCEPT____.

A. realism
B. idealism
C. structuralism
D. none

To determine the theory of international relations that does not have a pessimistic attitude, we need to understand the basic principles and perspectives of each theory.

A. Realism: Realism is a theory that assumes a pessimistic view of international relations. It emphasizes the existence of anarchy in the global political system, where states prioritize their own security and self-interest. Realism argues that international relations are inherently characterized by competition, conflict, and the pursuit of power.

B. Idealism: Idealism, also known as liberalism, does not have a pessimistic view of international relations. It assumes a more optimistic perspective and believes that progress and cooperation are possible in international politics. Idealism emphasizes the importance of international institutions, norms, and values to promote peace, democracy, and human rights.

C. Structuralism: Structuralism refers to a range of theories that emphasize the influence of broader social, economic, and institutional structures on international relations. Like realism, structuralism tends to have a pessimistic view of international relations. It argues that global systems and structures shape the behavior of states and often lead to inequalities and power dynamics.

D. None: This answer choice suggests that none of the theories listed have a pessimistic attitude toward international relations. However, based on the explanations above, both realism and structuralism can be considered theories that have a pessimistic view.

Therefore, the correct answer is B. Idealism, as it is the theory that does not have a pessimistic attitude toward international relations.