How did the assassination of Franz Ferdinand lead to Germany fighting against Russia?

The murder of Archduke Franz Ferdinand outraged AustriaHungary. Austria-Hungary was furious and with Germany's support, declared war on Serbia on July 28. Within days, Germany declared war on Russia—Serbia's ally and invaded France via Belgium, which then caused Britain to declare war on Germany.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."