Which identifies the effects of the congress of vienna?
the unification of germany led to the seven years’ war.
the harsh treatment of france led to a rise in nationalism throughout europe.
an interconnected europe led to the creation of the european union.
the congress system it set up became a model for the united nations.

The correct answer is "The congress system it set up became a model for the United Nations".

The Congress of Vienna was held between late 1814 and during 1815 and gathered ambassadors from all European states. The idea was to negotiate a viable manner of keeping a long lasting peace in the continent, specially after the French Revolutionary Wars and the Napoleonic works. Most of the participants were conservative politicians who intended to defend the status quo from the emergence of nationalistic and liberal revolutionary movements. It has actually beendescribed as a reactionary alliance in favour of returning the power to the hands of the absolut monarchies. This congress set the international political scenario until the outbreak of World War One.

It was actually the first time in history, in which representatives from the different states sat together and discussed agreements and treaties. Previously, they only had sent messages to each other, but they had never gathered. This is why it is said that Congress of Viena set the model for current international politics, and for the functioning of supranational organisms such as the United Nations.

The congress system if set up became a model for the United Nations.

Explanation:

My authority source for choosing answer C is the United Nations itself.  UN Chronicle, the magazine of the United Nations, featured an article titled, "From the Congress of Vienna to Present-Day International Organizations" (December, 2014).  That article asserts points such as these:

When did the process of international organization start?1 It was not in 1945 nor in 1919. Rather, it was the Congress of Vienna (1814-15) that proved to be the relevant turning point in history, when certain conditions allowed a number of European States to set in motion a series of innovations, inventions and learning processes that shaped the core of what we today refer to as international organizations (IOs).The major innovation at Vienna was the follow-up conference. This new idea resulted in the custom of participating States to convene, upon reaching an agreement, a follow-up conference to assess whether previously agreed-upon decisions and policies had been executed. ... Consequently, since 1815, that innovative idea resulted in an ongoing cycle of conferences dealing with similar and related issues. Apart from continuity, the cycle produced incremental decision-making and path dependency with regard to selected common solutions and efforts.

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