Which piece of evidence would best support the claim that "all new territories to the U. S. should decide for themselves whether they will be slave or free"?
The Missouri Compromise established a clear boundary for slave holding and non-slave holding states by creating the Mason-Dixon
line
The Compromise of 1850 established the precedent that new territories would choose for themselves whether to be slave or free.
The Tenth Amendment grants states powers not given to the Federal government by the Constitution or prohibited within the
Constitution
The Fugitive Slave Act gave the federal government the ability to assist slave owners in capturing fugitive slaves.

The piece of evidence that would best support the claim that "all new territories to the US should decide for themselves whether they will be slave or free" is the Compromise of 1850, that established the precedent that new territories would choose for themselves whether to be slave or free.

Explanation:

The Compromise of 1850 was an agreement between the different states of the United States regarding the status with which the different territories obtained after the war with Mexico would enter the Union. The question was whether these states would be free or slave, and how this would affect the balance between the two groups of states in Congress. Finally, through this agreement California was admitted as a free state, while Utah and New Mexico could define their status through popular sovereignty. The most important part of this agreement was the acceptance of popular sovereignty as the defining method of determining the status of the states against slavery. This would be applied again after the sanction of the Kansas-Nebraska Act, which would lead to a prelude to the Civil War in the event known as Bleeding Kansas.

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