Final answer:
When Benjamin Franklin was 21 years old, the English monarch who died and was succeeded by another was George I. His grandson, George III, was the king during the American Revolution.
Step-by-step explanation:
When Benjamin Franklin was 21 years old, in 1726, the English monarch who died and was succeeded by another was George I. George I was the first Hanoverian to take the throne of England. His grandson, George III, was the king at the time of the American Revolution.