Final answer:
The assassination of Archduke Ferdinand was the primary cause that led Russia to mobilize along the German border in support of Serbia, culminating in the outbreak of World War I due to a complex system of alliances among European powers.
Step-by-step explanation:
The primary cause that led to Russia mobilizing along the German border to support an alliance made with Serbia was the B) Assassination of Archduke Ferdinand. This event triggered a series of demands by Austria-Hungary upon Serbia, which, when not fully met, resulted in Austria-Hungary's declaration of war on Serbia.
Russia mobilized in defense of Serbia due to their treaty alliance and shared Slavic heritage. This in turn led to German mobilization to support Austria-Hungary and subsequently to declarations of war that expanded to include other nations due to the existing network of alliances. Germany's strategy also involved a preemptive strike on France through neutral Belgium, bringing Britain into the conflict based on their treaty to aid Belgium.
Russian mobilization was a direct response to the threat against Serbia, and the intricate web of alliances enabled a localized incident to escalate into the global conflict that became World War I. The plan to entangle Russia and France in a war, supported by the German Mobilization Plan, relied heavily on assumptions regarding the Russian military's slow mobilization and France's vulnerability, which proved to be miscalculations with grave consequences.