Final answer:
Gavrilo Princip was the Black Hand terrorist who assassinated Archduke Franz Ferdinand to start WWI.
Step-by-step explanation:
The name of the Black Hand terrorist that assassinated Archduke Franz Ferdinand to start WWI was Gavrilo Princip.
Gavrilo Princip was a member of the Serbian nationalist group, The Black Hand, that had been encouraged and supported by Serbian military officers. On June 28, 1914, Princip shot and killed Archduke Franz Ferdinand and his wife Sophie during their visit to Sarajevo.
This assassination was a key trigger for the start of World War I, as Austria-Hungary blamed Serbia for the attack and issued demands for restitution, which Serbia refused, leading to Austria-Hungary's invasion of Serbia and the activation the European alliance system.