Final answer:
Michigan, Minnesota, Iowa, and Oregon were Union states during the Civil War. These northern states supported the Federal Army in its efforts to maintain the Union. Tennessee, not listed among these states, seceded and joined the Confederacy.
Step-by-step explanation:
The states of Michigan, Minnesota, Iowa, and Oregon during the Civil War are best described as Union states. These states were part of the northern states that opposed the Confederate states which were in the south. Union states provided troops and resources to the Federal Army to preserve the Union and fight against the secessionist states which had formed the Confederate States of America.
As for the options provided for a question on which state was not a slave state that remained in the Union, the answer is a. Tennessee, because Tennessee seceded from the Union and joined the Confederacy. Other states listed such as Maryland, Delaware, and Missouri were indeed slave states but chose not to secede and remained with the Union.