The Answer:

Maryland.

The District of Columbia was established by congressional acts of 1790 and 1791 and selected by George Washington. It was originally a 10-mi (16.1-km) square (100 sq mi/259 sq km), with Maryland and Virginia granting land on each side of the river, including the town of Georgetown and the county of Alexandria respectively. The "Federal City" was laid out at its center. Alexandria county was returned to Virginia in 1847. The city continued to grow on the east bank of the river and in 1878, when Georgetown became a part of Washington (although it continued to operate as a separate city until 1895), the city of Washington and the District of Columbia became one and the same.

The Columbia Electronic Encyclopedia.

—The Fact Monster