Answer
$$\text{A}$$
Work Step by Step
The ladder, the wall of the building, and the ground form a right triangle.
(Refer to the attached image below.)
Recall:
The Pythagorean Theorem states that the square of the hypotenuse (the longest side) is equal to the sum of the squares of the two legs (two other sides).
The triangle has a hypotenuse of $30$ feet and a height of $24$ feet.
If we let $x$= the length of the base, then
\begin{align*}
30^2&=24^2+x^2\\
900&=576+x^2\\
900-576&=x^2\\
324&=x^2\\
\sqrt{324}&=x\\
\sqrt{18^2}&=x\\
18&=x
\end{align*}
Thus, the base of the ladder is $18$ feet away from the building.
