That's the question they use to weed people out who are too smart for the job. If you bring the deliberate typo to their attention, they disqualify you.
What's crazier is that A is also a possible answer, if the wall is 2' tall. But C and D aren't possible unless the wall is {4,5} + 16/21' tall. The fraction is 9.143" in case anyone is wondering. But the answer is completely ambiguous. What balcony wall is 1-2' tall?! That's a safety violation!
The length of the bottom wall is 20 feet and the sum of the lengths of the top walls is 17.5 feet, meaning there is an additional 2.5 feet of wall width across the two walls of the balcony. Assuming the wall is uniform thickness, that gives us dimensions of 1.25 x 10.5 feet, or 13.125 square feet.
/s