Final answer:
The main cause of the First World War was an intricate mix of Militarism, Alliances, Imperialism, and Nationalism, with no singular factor solely responsible for the outbreak. The correct option is A.
Step-by-step explanation:
The main cause of the First World War was a complex mixture of factors, best summarized by the acronym MAIN: Militarism, Alliances, Imperialism, and Nationalism. Though nationalism played a significant role, it was not the sole cause. Nationalism created a sense of pride and thirst for power among nations, desire for autonomy among ethnic groups, and ultimately, intensified competitions and rivalries between the Great Powers.
These elements, combined with the complex web of alliances, led to a situation where a single incident could trigger a chain reaction, as seen with the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist in 1914. This act sparked the outbreak of WWI, as countries were bound by treaties to defend each other, setting a global conflict in motion.