The horizontal legs of the upper triangles are equal in length to the vertical leg (2.5 m) since those triangles are isosceles right triangles.
The overall area is half the product of the lengths of the diagonals, so is
.. A = (1/2)(2*2.5 m)(2.5 m +6 m)
.. = (2.5 m)*(8.5 m)
.. = 21.25 m^2
The 3rd selection is appropriate.