The international system after 1815 was called the Concert of Europe.  The correct answer is B. The Holy Alliance was a conservative agreement between three dominant Powers_i.e. Austria, Prussia and Russia_to keep the monarchical nature of the international system. The Westphalian System refers to the previous period before The Concert of Europe. The Long Peace refers to the time period lasted until the outbreak of World War I. Congress of Vienna was held 1814–15 to agree the settlement of Europe after the Napoleonic Wars and marked a return to a Europe composed of monarchies.