Final answer:
The Balkans were known as the "powder keg" due to intense nationalism and the competition between the weakening Ottoman Empire and Austria-Hungary, coupled with Serbian ambitions to unite Slavic nationalities under its rule. These factors created a volatile and tense environment that ultimately contributed to the outbreak of World War I.
The correct option is D.
Step-by-step explanation:
The term "powder keg" used to describe the Balkans refers to the region's high levels of tension and instability, which were akin to a keg of gunpowder that could ignite and explode at any time. This volatility was largely due to intense nationalism among different ethnic groups wanting to form their own independent nations and the fading power of empires like the Ottoman Empire and Austria-Hungary vying for control over the region.
With the presence of various Slavic nationalities and religions, as well as the territorial ambitions of Serbia to unite the separate nationalities under a single Slavic identity, the Balkans became a hotbed of political activities. The pan-Slavic nationalism pushed forward by Serbia received support from Russia, creating a further divide with Austria-Hungary, which saw Serbian ambitions as a threat to its own rule in the region. The Ottoman Empire's decline exacerbating the situation, as Muslims were forced out, led to a demographic change and a power vacuum that competing nationalities sought to fill.
All these factors contributed to the Balkans being known as the "powder keg of Europe" due to the potential for conflict that could, and eventually did, spark a wider war - World War I.