Final answer:
Louis the Pious was the Frankish king whose throne was disputed. The dispute over the throne led to the Treaty of Verdun in 843.
Step-by-step explanation:
The Frankish king whose throne was disputed was Louis the Pious.
After the death of Charlemagne, Louis the Pious became the sole ruler of the Frankish Empire. However, his three sons, Lothair, Louis the German, and Charles the Bald, challenged his power and engaged in a series of civil wars known as the Carolingian Civil War.
This dispute over the Frankish throne ultimately led to the Treaty of Verdun in 843, which divided the Frankish Empire into three separate kingdoms.
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