Answer is : B. Russia
The Austrian Empire was worried since the Russian Empire was promoting a pan-Slavist policy in Eastern Europe and the Balkans. This policy was designed to unite all peoples with roots historically known as the Slavish nations. The tension became to increase when Austria and Russia were taking control by establishing posts of influence in areas near the Ottoman Empire. Russia influence was seen as a threat, since the Ottoman Empire had Christian Orthodox minorities that were seeking protection from the Tsar.
In the Balkans, it became clear that Germany and Austria as well formed a block that tried at all costs to prevent the emergence of a large, multinational Slavic block in the region. (we can see the later formed Yugoslav state as an example where former Ottoman influence vanished, and relationships with Russia later became more important as with Austria).
