Final answer:
The answer to the student's question is c) Harald III, also known as Harald Hardrada, who became the sole king of Norway after his brother Magnus' death in 1069.
Step-by-step explanation:
On his brother Magnus' death in 1069, Harald III Haraldsson became the sole king of Norway. Therefore, the correct answer is c) Harald III. Harald III, also known as Harald Hardrada, was a prominent figure in the history of Norway and was known for his attempts to claim the English throne, which ended with his death at the Battle of Stamford Bridge in 1066. This period was marked by various important historical events, including Viking explorations and the movement towards Christianity in the Scandinavian region. The runestone of Gorm and Harald Bluetooth's subsequent stone, both in Jelling, Denmark, are significant artifacts from this era, documenting the conversion to Christianity and unification efforts.