Explanation:

(Refer to th efigure below)
Since the N and S walls are parallel, we can say △DFB and △DEC are SIMILAR triangles.

i.e. \(\frac{DE}{DF} = \frac{DC}{DB}\)
\(\frac{30}{60} = \frac{DC}{DC + 15}\)

\(DC + 15 = 2DC\)
\(DC = 15\)

This makes △ABD as an Equilateral triangle with each side \(30ft\)

Area of an Equilateral triangle = \(\frac{\sqrt{3}}{4}(side^2) = \frac{\sqrt{3}}{4}(30^2) = \frac{900\sqrt{3}}{4} = 225\sqrt{3}\) \(ft^2\)

Col. A: \(225\sqrt{3} ≈ 390\)
Col. B: \(350\)

Hence, option A