What best defines internationalism?
the build up of armaments
political and social reforms
active involvement in world affairs

Internationalism refers to the idea that we all are, and should be, part of a broader community, not just the nation or the state. Therefore it has focused in the areas of universal government and an universal language. As you can see, the correct answer is "active involvement in world affairs". It seeks cooperation among the different countries and their people.

Although it is a  concept that is under constant discussion, this common points of view often appear in it.

the following what exactly?

a simple sentence must have at least one dependent clause.