‘german foreign policy was unreasonably aggressive from 1880 to 1914.’ show how far the evidence confirms this statement.

In the late 1800s, leading up to 1914, Germany aggressively pursued a policy of becoming a global power.  The German navy was increased exponentially, to the point that it could be a rival to Britain's dominance on the seas.  This increasing militarization in Germany was unreasonable since it only propelled Europe closer and closer to war.
German also aggressively pursued an empire in Africa and Asia, but the effort and expense on these ventures exceeded their value to Germany.  (Not to mention the unreasonable treatment of native populations.)
Germany pursued aggressive foreign policies to an unreasonable degree ... but so did the rest of Europe during those years.  Thus the outbreak of World War I was being spurred on by the increasing militarization and foreign policy tension between the great powers.