c) John C. Frémont

John C. Frémont, an American military officer and explorer, played a key role in the early exploration of California during the Mexican-American War. Known for his comprehensive surveys and mapping of the Western territory, Frémont’s expeditions contributed significantly to American understanding of these vast areas. His actions in California during the war, including his involvement in the Bear Flag Revolt, were pivotal in the U.S. acquiring California. Frémont’s expeditions and military service were instrumental in expanding the nation’s borders and in shaping the American West.