look at the lower right triangle (the horizontally-positioned one)
the hypotenuse is the ladder, which is 26 feet long, the longer leg is 24, so we can find out the shorter leg by using the Pythagorean theorem, which is
√(26²-24²)=10
after sliding down 14 feet, the ladder top end of the ladder is still 10 feet above the ground, which means originally the top end of the ladder was 14+10=24 feet from the base of the building.
now focus on the standing-up right triangle,
the hypotenuse is 26, the vertical leg is 24, so the horizontal leg is 10
the answer is a.