GMAT Sentence Correction (SC) | Re: QOTD: Although the Ottoman Turks under the leadership of

The complete C would look like:
C) Although the Ottoman Turks under the leadership of Suleiman II had taken control of Rhodes and many of the largest Mediterranean islands,their unsuccessful siege of Vienna in 1529 marked the beginning of the long decline of their empire, stopping the advance of Islam into central and western Europe and ensuring that the Christian rather than the Muslim religion and culture would dominate the region.

I am still struggling how "their" refers back to
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