The correct answer is B) the political power of those who supported slavery.
The statement that could be considered a good definition of what "slave power" was as far as the North was concerned is " the political power of those who supported slavery.
In 1850, people from the North were afraid of the political power plantation owners of the South could have with slavery. They considered that slaveholders of the South had social, economic, and political influence due to the slavey system in place in the Confederated States. These powerful Southern people -a minority- could influence important decisions in government and maintained the use of slaves in their plantations.