Final answer:
Janos Hunyadi ended Vlad II Dracul's rule due to political struggles and Vlad II's ties with the Ottomans, rather than a specific event like a popular uprising or illness.
Step-by-step explanation:
Janos Hunyadi terminated Vlad II Dracul's reign in November 1447 primarily due to political power struggles, including disagreements with Vlad II and the broader context of Ottoman influence in the region.
Hunyadi, a powerful Hungarian military leader and statesman, perceived Vlad II Dracul's alliance with the Ottomans as a threat to Christian Europe and his power. Vlad II had previously been a vassal of the Ottoman Empire, which put him in a precarious position amidst the power plays between Christendom and the Ottoman expansion.
The termination of Vlad II's reign cannot be directly attributed to a single factor such as a popular uprising, illness, or the specific historical events listed in the question (depopulation due to the bubonic plague, the death of the Byzantine emperor, or the Ottoman victory at the Battle of Kosovo), as these occurred either before or after the lifetime of Vlad II Dracul and Hunyadi. Instead, it was a calculated political move by Hunyadi to secure his boundaries and strengthen Christian resistance against the Ottomans.