Answer: Germany.
Some extra relevant information:
During the 17th to 19th centuries, the Kingdom of Prussia was primarily located in the territory of what is now modern-day Germany. Prussia emerged as a significant power in Europe under the rule of the Hohenzollern dynasty, expanding its influence and territorial control through strategic alliances and military conquests.
Originally centered in the northeast of the Holy Roman Empire, Prussia gradually grew in size and importance. Its capital, Berlin, became a bustling metropolis and the heart of the kingdom. Throughout its history, Prussia annexed various territories, including parts of Poland, Lithuania, and Denmark, which eventually formed the provinces within its boundaries.
By the late 19th century, Prussia consolidated its leadership in the German Confederation and played a crucial role in the unification of Germany under Emperor Wilhelm I in 1871. This newly formed German Empire included the Kingdom of Prussia as its leading state.
In summary, the Kingdom of Prussia was predominantly situated in what is now Germany, with Berlin as its capital, and it played a pivotal role in shaping the future of the country.