At the outbreak of the Civil War in 1861, pro-Union sentiment was strong in western Virginia, eastern Tennessee, and western North Carolina primarily because?

Fewer slaves and Fewer large plantations were found in these areas

Explanation:

the outbreak of the American civil war in 1861 was majorly because of the unending disagreements between the Union of the North and the confederate states of south over slavery, state rights and the expansion westward.  

The Union was strongly against slavery and the expansion of slavery into new territories as proposed by the south. the south found this opposition worrying because they have built their economy on the use of slave labor to grow agricultural produce for commercial purpose. the continues disagreements finally led to the American civil war in 1861.

pro- union sentiment was strong in Western Virginia, eastern Tennessee and Western North Carolina because there were few slaves and smaller plantations in the regions.