答案：E

解析：

The ladder, the wall, and the ground form a right triangle with a 25-foot hypotenuse. At first, the bottom of the ladder is 7 feet from the base of the building, so one leg of the right triangle measures 7 feet; the length of the other leg, x, can be found by solving (7^2) + (x^2) = (25^2), which is the Pythagorean theorem. From this, you can figure out that the other leg measures 24 feet.

After the ladder slips down 4 feet, the 24-foot leg of the right triangle becomes 20 feet long. The other leg then has to be 15 feet long. This length is found by solving (20^2) + (y^2) = (25^2), which is again the Pythagorean theorem.

Since the distance between the bottom of the ladder and the base of the building increases from 7 feet to 15 feet, the amount that the bottom of the ladder slides out is 8 feet.