You must be talking about the assassination of Archduke Franz Ferdinand of Austria. This was an event which triggered the start of WWI.
This is a hard one... but I believe it is either C or D. We know that wounded Allied soldiers were a big issue during the war and a big reason as to why the U.S. joined was to gain land and protect its allies that it related to (mainly being Britain). D is also a reasonable answer, but I highly doubt it is the other options. Double check your history book, but I think the answer is C.
