Which identifies the effects of the congress of vienna?

Is this question multiple choice? It sounds like it should include the answers. The Congress of Vienna redrew the map of Europe in order to restore the old monarchies in places Napoleon conquered. 

Based on answer choices I've seen elsewhere, here's the answer I'll suggest:

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.