The correct answer is - the Kingdom of Serbs, Croats, and Slovenes.
The initial name of Yugoslavia, was actually the Kingdom of Serbs, Croats, and Slovenes, or as it is often referred to in the Balkans the Kingdom of SHS. This mane was largely given to the country because of the three dominant ethnic groups in it, the Serbs, the Croats, and the Slovenes.
Afterwards, with the inclusion of Macedonia in it, the name changed in Yugoslavia, and also Bosnia and Herzegovina, and Montenegro, were given bigger recognition as federal states. All of the states had their names and the prefix SR (meaning Socialist Republic) in front of their names.
Yugoslavia was a name given because it means the land of South Slavs, with yug/o meaning south, and slavia meaning the land of Slavs.
