Austrian-Serbian hostilities initiated World War I.

a. True

b. False

True.

After the Archduke Franz Ferdinand was murdered, the Austrian Empire declared war on Serbia on 28 July 1914.

This is true. What happened at the time was that a Bosnian anarchist who was present in Serbia at the time assassinated the at-the-time ruling Archduke Ferdinand. This lead, through a short cascade of events, to an all out war in the Balkans and later the whole world.