As shown in the picture above, you are given an opening of the brick with a combination of a rectangle and a semicircle. To find the area of the total opening, you need to have the formula of the area of the rectangle and the semicircle.
A (rectangle) = LW
A (rectangle) = (4ft) (2.5ft)
A (rectangle) = 10 ft²
A (parabola) = 2/3ab
A (parabola) = 2/3 (1.25) (4ft)
A (parabola) = 5 (2/3)
A (parabola) = 3.33ft²
A (total area) = rectangle + parabola
A (total area) = 10 + 3.33
A (total area) = 13.33ft²
The answer is letter D.