We are given the area of the pool with the tile included, as well as the size of the tile border.

So with the tile border, the area of the pool is $225π$, meaning that the length of the radius is $15$ (because the area of a circle is $πr^2$, we take the square root of $225$ to find the radius). $\sqrt{225} = 15$.

If the radius of the pool and tile is $15$, and the tile border is $3$ feet wide, then the radius of the pool alone is $15 - 3 = 12$.

With this info, we can find the area of the pool alone using the formula for the area of a circle.

Area of circle $= πr^2$
$π12^2 = 144π$ = area of the pool

Answer D