Final answer:
The Kansas-Nebraska Act is the federal legislation that formally embedded Douglas's opinions on how slavery should be addressed in the territories.
Step-by-step explanation:
The correct answer is c) The Kansas-Nebraska Act.
Douglas's opinions on how slavery should be addressed in the territories were formally embedded into the Kansas-Nebraska Act. This act, introduced by Stephen Douglas in 1854, proposed to create two territories and supported the use of popular sovereignty, allowing the residents of these territories to decide for themselves whether to allow slavery. It also repealed the Missouri Compromise line, which angered many northerners who opposed the extension of slavery into new territories.