Answer :
Final answer:
The 'Pathfinder', son-in-law to Senator Thomas Hart Benton, was John C. Frémont, a significant figure in mid-19th century explorations of the American West.
Explanation:
Senator Thomas Hart Benton's son-in-law known as the 'Pathfinder' was John C. Frémont. He earned this nickname due to his extensive exploratory expeditions across the American West during the mid-19th century. His accomplishments included being one of the first white men to chart the entirety of the Oregon Trail, as well as his prominent role in the American conquest of California during the Mexican-American War.
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