Final answer:
The best example of an international alliance is B, Canada and the United States making an agreement to support each other in case of attack, which aligns with the concept of military alliances like NATO. Trade agreements such as USMCA and the EU, as well as the WTO's role in international trade, are different forms of international cooperation.
Step-by-step explanation:
The best example of an international alliance in the options provided is B: Canada and the United States make an agreement to support each other in case of attack. This option specifically refers to the type of military and defense alliance that countries enter into for mutual support, much like the North Atlantic Treaty Organization (NATO). NATO is a significant example of such an alliance, as it was established to offer collective defense against aggression and has a wide array of member states which agree to mutual defense in response to an attack by any external party.
Trade agreements such as NAFTA, rebranded as the United States-Mexico-Canada Agreement (USMCA), and the European Union (EU), are distinct from military alliances, focusing instead on economic collaboration and the integration of economic policies. The World Trade Organization (WTO) works to facilitate and regulate international trade, addressing disputes that arise among member nations.