Who was The President most responsible for obtaining the lands of California Oregon and New Mexico US?

The correct answer is James K. Polk, who served as the 11th President of the United States from 1845 to 1849.

His administration is primarily known for expanding US territory, particularly on the West Coast and Southwest through territorial acquisition and negotiation during The Mexican American War(1846-8). As a result, The US gained significant land areas including modern-day California, Oregon, and New Mexico