Why were Delaware, Maryland, Kentucky, and Missouri considered border states during the Civil War?

Abecause they had boundaries on the ocean
Bbecause they were slave states that didn't secede
Cbecause they were free states that seceded
Dbecause they were free states next to slave states

Verified Answer :

The reason why Delaware, Maryland, Kentucky, and Missouri were considered border states during the Civil War is "b. because they were slave states that didn't secede" from the Union, since these were vital states to "win over" for the Union to win the war. 

BThey were slave states that did not seceded.