Which international agreement focuses on military alliance and mutual defense among member countries?

The answer is correct because NATO, or the North Atlantic Treaty Organization, is specifically centered around military alliance and mutual defense. Established in 1949, NATO is a collective defense organization where member countries agree that an armed attack against one or more members is considered an attack against them all. This principle of collective defense is enshrined in Article 5 of the North Atlantic Treaty, which forms the foundation of the alliance's mission. The organizational structure and operational principles of NATO are designed to ensure security and cooperation among its member states in the realm of defense and military operations.

In contrast, the other options relate to different contexts. The Treaty of Versailles is primarily known for ending World War I and imposing reparations and territorial losses on Germany, rather than establishing a military alliance. The Geneva Conventions set the standards in international law for humanitarian treatment in war, focusing on the protection of those who are not participating in hostilities. Lastly, the Paris Agreement relates to climate change and global environmental efforts, not military or defense matters. Hence, NATO is uniquely positioned as the international agreement that directly addresses military alliances and mutual defense among its member countries.