D. Provide the Ottoman Empire with trained professional soldiers
Explanation:
The Devshirme system was a system through which the Ottoman Empire was getting recruits for its military. This system was also known as the ''blood tax'' and the people of the Balkan Peninsula had it the hardest with it. The Ottoman officers were going each year and getting the strongest young boys, usually preferring to take them at age 8 or 9, but it was done with boys up until the age of 18. Once taken away, the little boys were trained for the military, as well as being thought to practice the Islam and the Sultan, thus making them a loyal military force. The name used for this soldiers was mostly Yanichari.
I don’t know if this is right but as someone who is native I’m going to guess during conversion of religion. Again I don’t know if that’s the main one they want but it was a contributing factor. :)