Final answer:
The state with a circular northern boundary with Pennsylvania is Delaware, resulting from the resolution of a border dispute settled by the Mason-Dixon Line survey in the 1760s.
Step-by-step explanation:
The state with a northern boundary with Pennsylvania that is circular is Delaware. It was the result of a historic border dispute involving Pennsylvania's proprietor, William Penn, and Lord Baltimore of Maryland. The dispute was due to the charter granted by King Charles II, which was vague concerning the northern and southern borders of the Pennsylvania colony.
This disagreement remained unresolved until the Mason-Dixon Line was surveyed in the 1760s, which famously included a circular boundary known as the Twelve-Mile Circle at Delaware's northern end. This circle became part of the final resolution of the border dispute and defined the present-day boundaries between Pennsylvania, Maryland, and Delaware.