What circumstances might have induced the German emperor to pledge to "faithfully stand by" Austria-Hungary and its emperor?

On July 5th, 1914 Germany pledged faithfully to stand by Austria-Hungary, also called a Blank Check. This was a major event of WWI and a huge error by Germany. They pledged to back any action taken by Austria-Hungary regarding Serbia.

One consequence that might have induced Germany to give the blank check was that Emperor Franz Josef defended that the only way to balance power in the Balkans was to punish Serbia, which was pictured as a major threat to the Empire. Franz Josef also pictured Russia’s economic policy as a threat to the German Empire.

the answer is definitley b!