To determine how many feet of board Mia needs to cut off, we need to know the current width of the third board and the desired width to match the shelter.
Assuming the given options correspond to common cuttings that might be necessary, we can deduce the correct answer. If the requested width is 6 inches less than the current width, then Mia would need to cut off 6 inches or 0.5 feet.
If you can provide the current width of the board and the width Mia needs to match, I can give you a definitive answer on how much she needs to cut off.