Final answer:
The capital of the Kingdom of Serbs, Croats, and Slovenes in 1919 was Belgrade, which later remained as Serbia's capital after the breakup of Yugoslavia.
Step-by-step explanation:
In 1919, the city that became the capital for the Serbs, Croats, and Slovenes was Belgrade (A). After World War I, the region previously known as the Kingdom of Serbs, Croats, and Slovenes formed a new country that was later called Yugoslavia. This multinational state was created out of the Austro-Hungarian Empire's territories, predominantly inhabited by South Slavic peoples. With the breakdown of Communism towards the end of the 20th century, the Yugoslavian federation eventually disintegrated into separate nations, escalating into violent conflicts along ethnic and religious lines. The capital city, Belgrade, later continued as the capital of Serbia.