To Ensure the Smooth Transition of Authority After Him, Charlemagne

To ensure the smooth transition of authority after him, Charlemagne employed what means of succession?

A) He had his only surviving son crown himself as co-emperor.
B) He wrote an explicit cartulary that designated his heir and second choice.
C) He authored the Treaty of Verdun.
D) He divided his territory among his sons but bestowed the title of Holy Roman Emperor on the oldest son.
E) He allowed the pope to choose the next successor.