Final answer:
The effect of the ultimatum given to Serbia by Austria-Hungary in July 1914 was the start of World War I.
Step-by-step explanation:
The effect of the ultimatum given to Serbia by Austria-Hungary in July 1914 was A) The start of World War I. Following the assassination of the Austrian Archduke Franz Ferdinand, Austria-Hungary issued a list of demands to Serbia. Germany supported Austria-Hungary and when Russia declared war on Austria-Hungary, Germany declared war on Russia, and then France and Great Britain got involved. This chain of events led to the start of World War I.