Which action during westward expansion had the most negative impact on the lives of slaves? A. The Fugitive Slave Act
B. The Mexican-American War
C. The Treaty of Washington
D. The Missouri Compromise of 1820

A. The Fugitive Slave Act

Explanation:

A is correct as it allowed slave owners to reclaim runaway slaves.

B is also not correct as after the war California entered Union as a free state and many other state had the opportunity to choose whether they would prohibit slavery.

C is not correct as it ended the slave trade.

D is not correct as according to it slavery was prohibited north of the southern boundary of Missouri.

the answer here would be