The correct answer is A) unrestricted submarine warfare, American ultimatum, moratorium, cancellation of the Sussex Pledge.
The order of events that describes changes in Germany's policy of unrestricted submarine warfare was: unrestricted submarine warfare, American ultimatum, moratorium, cancellation of the Sussex Pledge.
In 1915, Germany introduced the unrestricted submarine warfare in WWI. In this moment, Germany declared the area around the England Isles as a war zone. Germany clearly stated that all merchant ships from the region or those ships from neutral countries would be attacked by the German navy if they sailed across the England Islands war zone.
In 1916, during World War I, Germany made a promise to the U.S. called the Sussex Pledge before the U.S entered the war. The promise was that Germany would change the 1915 naval policy and would not attack passengers ships in the zone and merchant ships would not be sunk until there was a confirmation that the ship carried arms.