They blame it on Serbia. It was their terrorist organization called the Black Hand that killed him (and his wife).
The archduke Ferdinand was assassinated in Sarejevo in Bosnia/Herzegovina.
Austria-Hungary blamed Serbia for the assassination. The assassination happened in Bosnia, which was ruled by Austria-Hungary at the time; Bosnian separatists murdered the Archduke with some assistance from Serbian intelligence officials. The Austrian government, of course, found out about this and was not too happy.
No single country is to blame.
Austria
Yugoslavia
sophie
Austria
Austria-Hungary
Austro-Hungary
Serbia
the answer is A:austria-hungary