Final answer:
The correct answer to the historical query about the slave state that did not remain in the Union during the Civil War is Tennessee, as Maryland, Delaware and Missouri were border states that stayed with the Union.
Step-by-step explanation:
The student inquired about where Dill lives during the school year, with options including Arkansas, Mississippi, Georgia, and Boston. However, this question seems to be mixed up with another historical query related to the Civil War.
To clarify, the statement 'All of the following were slave states that remained in the Union except' refers to border states during the American Civil War. Here, the correct answer is a. Tennessee, as it was the only one among the options provided that seceded from the Union and joined the Confederacy. Maryland, Delaware, and Missouri were slave-holding states that remained in the Union.