To apply the Pythagorean Theorem in this situation, we can consider the right triangle formed by the height line, the slant height line, and part of the base.
Let the height, h, be the length of the height line. Let the slant height, L, be the length of the slant height line. Let the half of the base, b, be the length of the right base edge.
According to the Pythagorean Theorem, we have:
h^2 + (b/2)^2 = L^2
Substituting the given values, we have:
h^2 + (40/2)^2 = 25^2
Simplifying the equation:
h^2 + 20^2 = 625
h^2 + 400 = 625
h^2 = 625 - 400
h^2 = 225
Taking the square root of both sides:
h = √225
h = 15
Rounding to the nearest tenth, the height of the square pyramid is 15 feet.
Therefore, the correct answer is B) 15.0 ft.