Final answer:
The primary source was created just after Austria-Hungary declared war on Serbia in 1914.
Step-by-step explanation:
The primary source in question was created just after Austria-Hungary declared war on Serbia. This declaration of war occurred on July 28, 1914. The source also mentions that all of Europe began preparing for war at that time, so option C, 'Just after Austria-Hungary declared war on Servia (Serbia),' is the correct answer.