Final answer:
The Triple Alliance became known as the Central Powers when Bulgaria joined, alongside Germany, Austria-Hungary, and the Ottoman Empire during World War I.
Step-by-step explanation:
When Bulgaria joined the alliance that previously consisted of Germany, Austria-Hungary, and Italy during World War I, the coalitions had already shifted. Italy had left the alliance and the Central Powers were then formed, which included Germany, Austria-Hungary, the Ottoman Empire, and later on Bulgaria.
This reformed alliance squared off against the Allied Powers, which included countries like Great Britain, France, Russia, and eventually the United States.
The addition of Bulgaria to the Central Powers strengthened their position in the Balkans, coming after the country had been defeated in the Second Balkan War and sought new alliances to regain territory and status.