The Ottoman Empire’s decline is conventionally dated from 1683, after the failed Second Siege of Vienna.
Ottoman forces reached Vienna in July 1683 under Grand Vizier Kara Mustafa Pasha, but the siege failed when a relief army led by John III Sobieski of Poland and allied imperial forces arrived on 12 September. The defeat ended the empire’s major westward drive into Central Europe and shifted the strategic initiative toward its Habsburg and Polish opponents.
The loss did not make the empire collapse overnight. The ensuing Great Turkish War continued until 1699, when the Treaty of Karlowitz forced the Ottomans to surrender extensive European territory, including most of Hungary. For that reason, 1699 is sometimes discussed as the year of a major territorial and diplomatic turning point.
The quiz’s wording asks when decline began after the failed siege, making 1683 the standard answer. The year 1529 refers to the earlier Ottoman siege of Vienna, which also failed, while 1717 belongs to a later phase of Ottoman-Habsburg conflict.