What does this document suggest is an underlying cause of world war one?

Britain entered  WWI under a moral and legal obligation to protect Belgium, under the treaty of London signed in 1839. After the German invasion of Belgium in 1914, Britain decided to come to the aid of Belgium and France and subsequently declared war on Germany.Britain suffered horrendous casualties, lost much of its vast wealth, and surrendered her paramount position as  the leading power of the world. But without Britain's effort, Germany would have dominated western Europe. Therefore, for moral, legal and  strategic reasons, Britain's participation in the first world war was justified.

Assassination of Archduke was the event that sparked the outbreak of World War I. The correct option among all the options given in the question is option "4". The assassination of Archduke Franz Ferdinand occurred on 28th July of the year 1914. Franz Ferdinand was the heir to Austro Hungarian Empire and he was shot dead along with his wife by Gavrilo Princip in Sarajevo. Gavrilo was a Bosnian Serb nationalist. This incident is considered the main reason behind the outbreak of the first world War. The war left around 17 million dead and about 20 million people wounded.