To determine the appropriate length board Mia should use for the length of the shelter, we need to convert 3.25 feet into inches.
Since there are 12 inches in a foot, we multiply:
3.25 feet × 12 inches/foot = 39 inches.
Therefore, Mia should use a 39 inch board for the length of the shelter.
So the correct response is: 39 inch board.