The correct answer is C) highly disciplined Janissaries.
The other options of the question were A) an enormous army. B) a powerful cavalry. D) innovative tactics.
The key feature that helped the Ottoman military defeat other armies in the region was highly disciplined Janissaries.
As we know, the Ottoman Empire was one of the most powerful empires of its time. It had a great and successful army that conquered many territories. Part of that success was due to the highly disciplined Janissaries that were the unit of elite infantry of the Ottoman Empire army. Emperor Murad I was the first to order the creation of this infantry elite group that also served as his bodyguards.