Final answer:
The treaty that ended the Great Northern War is Nystadt.
Step-by-step explanation:
The treaty that ended the Great Northern War is a. Nystadt.The Treaty of Nystad, signed in 1721, effectively ended the Great Northern War (1700–1721) between Sweden and a coalition of Russia, Poland, and Denmark. Option (a) Nystadt is correct. The treaty was negotiated in the town of Nystad (now Uusikaupunki) in Finland. The terms of the treaty marked a significant shift in the balance of power in Northern Europe. Sweden ceded several territories to Russia, including Ingria, Estonia, and Livonia, while also acknowledging the incorporation of Finland into the Swedish realm. This treaty reshaped the geopolitical landscape and solidified Russia's emergence as a major European power under the rule of Peter the Great.