During the Great War, other central power nations that joined the Triple Alliance were the Ottoman Empire, and Bulgaria. In the case of your question, the answer will be Bulgaria.
Explanation: The Triple Alliance consisted of Austria Hungary, Germany and Italy as a secret agreement in 1882. However, Italy remained neutral in 1914, and entered the WWI as an opponent of Germany and Austria Hungary. The Triple Alliance or the Central Powers, joined the Ottoman Empire and Bulgaria. Romania and Greece were on the side of the Allies, the so-called Triple Entente, while Switzerland was neutral.
