Final answer:
The European balance of power system ultimately led to continuous conflicts (Option A), as the alliances and boundaries designed for stability often resulted in tension and wars, including World War I.
Step-by-step explanation:
The European balance of power system, which was designed to maintain stability and prevent any single nation from dominating the continent, can be summarized as having led to continuous conflicts (Option A). Despite the intentions of creating a stable and peaceful Europe, the alliances and political boundaries established to maintain this balance often led to tensions, arms buildups, and the eventual outbreak of wars, including the Great War, known as World War I.
The period before World War I, known as the Long Peace, saw relative stability, but it was fragile and often disrupted by various military conflicts. Additionally, the structure of alliances created scenarios where regional conflicts could transform rapidly into major wars.
While the Concert of Europe managed to absorb some internal conflicts and maintain the status quo for a time, nationalism and liberal ideas were growing stronger and eventually contributed to the breakdown of the balance of power. Furthermore, the system of sovereignty and non-interference, while allowing for self-determination, also led to mass deaths in the twentieth century through wars and genocides.
In conclusion, while the balance of power system sought to prevent any single nation from becoming too powerful, it did not ensure lasting peace (Option B) or encourage isolationism (Option D), and the connection between this system and economic instability (Option C) is less direct and thus not the best summary of its effectiveness.