The correct answer is - B. They had pledged to fight with other countries.
After the assassination of archduke Ferdinand in 1914 lots of countries quickly joined World War I because they had already pledged to fight with other countries. The terrain for war was pretty much set, and alliances were already made, the interests were known and on the table for few years before that, so the assassination of archduke Ferdinand was just the point that marked the starting point of the almost inevitable war.
