In what way have nations joined forces to ensure peace

By forming various organizations.

The European Union, for example, unites Europe (Minus Britain these days) under one banner, thus preventing fighting.

A more direct example is the North Atlantic Treaty Organization. NATO protects the "North Atlantic" and Europe from terrorists and invading countries by using the combined military strength of multiple nations.

Start with the 1648 Treaty of Westphalia that ended the 30 Years War. This is the date historians place as the birth of the international system.

Move to the Congress of Vienna in 1815 which ended the Napoleonic Wars. It reasserted principles of state sovereignty.

Both of these important events dovetail into the Treaty of Versailles, which ended WWI and helped establish Woodrow Wilson's vision of a League of Nations (1918). Actual, tangible examples of joint international forces are the Allied Powers in both WWI and WWII that defeated the Austria-Hungarian Empire and the Axis Powers, respectively.

In the world that emerged after 1945, the most important development in international peace was the United Nations, whose charter was signed in San Francisco in Summer 1945. The second, NATO defense accord signed April 4, 1949 in Washington DC. Both gave the best 2 answers to international security and stability.

Of course, if you are also talking about economic peace in the form of cooperation, that's a whole other ball of wax. You have to look at the GATT, IMF, and World Bank in 1945.

I’m pretty sure that the answer is b.