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.

Sadly, not one of the allowed options.