Final answer:
The development of nation-states in Europe in the latter half of the nineteenth century is the factor that contributed to the outbreak of World War I. Nationalism led to the creation of unified countries, heightened rivalries, and complicated alliances which eventually escalated into the war following the assassination of Archduke Franz Ferdinand.
Step-by-step explanation:
The emerging factor in the second half of the nineteenth century that contributed to the beginning of World War I was the development of nation-states in Europe (Option A). This period saw the decline of empires and the rise of nationalism, where people with a common national identity sought political autonomy and unity, leading to the formation of nation-states. This surge in nationalism exacerbated tensions and fostered rivalries among emerging and established European powers, particularly in the context of militarism, alliances, imperialism, and colonial competition.
These nationalistic aspirations, combined with military strategies that relied on rapid mobilization and a tangled system of alliances, set the stage for a continental conflict. The assassination of Archduke Franz Ferdinand of Austria by a nationalist in Serbia acted as the catalyst for the war. Nationalistic desires not only in Europe but also in colonized territories further complicated international relations and fueled the expansion of the conflict that would become the first global war.