Final answer:
The largest single addition to American territory was the Louisiana Purchase, which doubled the nation's size and had a profound impact on the country's expansion and development during the 1800s. The correct answer is option a.
Step-by-step explanation:
The largest single addition to American territory was the Louisiana Purchase. This significant event took place in 1803 when Thomas Jefferson signed the Louisiana Purchase, effectively doubling the nation's size. For $15 million, the United States obtained the land west of the Mississippi River from the Gulf of Mexico to the Canadian border, which was a mostly unexplored territory inhabited by Native Americans. This acquisition started a massive westward expansion, shaping the country's growth during the 1800s.
The importance of the Louisiana Purchase is underscored by the fact that it alerted the course of American history, contributing to the Manifest Destiny ideology and initiating ventures like the Lewis and Clark expedition. This monumental land deal provided the United States with more agricultural opportunities, spurred the migration westward, and laid the groundwork for future settlements and statehoods.
Comparatively, the Mexican Cession and the acquisition of the Oregon Country were substantial but did not match the size and impact of the Louisiana Purchase. Likewise, the Old Northwest and Alaska were significant but not as extensive in terms of acreage added at one time.