Answer:
A. Russia mobilized its troops.
Explanation:
On July 28, 1914, a month and a day after the Archduke Franz Ferdinand of Austria and his wife were murdered by a Serbian nationalist in Sarajevo, Austria-Hungary declares war on Serbia, a moment that marks the effective beginning of the First World War.
Threatened by Serbia's ambition in the Balkan region, the Austro-Hungarian Empire determined that the appropriate response to the killings was to prepare for a possible military invasion of Serbia. After securing the unconditional support of its powerful ally, Germany, Austria-Hungary presented Serbia with an ultimatum on July 23, 1914, demanding, among other things, that all anti-Austrian propaganda within Serbia be suppressed, and that Austria -Hungary could carry out his own investigation into the assassination of the Archduke. Although Serbia complied with all of Austria's demands except for one, the Austrian government broke diplomatic relations on July 25 and went ahead with military readiness measures. Meanwhile, faced with the impending crisis, Russia, Serbia's powerful ally in the Balkans, took the first steps towards military mobilization against Austria. In a short time, entire Europe entered the armed conflict.
