The formula for the area of a circle is:
A=πr²
We also know that the diameter of a circle is twice the size of its radius
d=2r
So if the diameter is 18...
18=2r
divide both sides by 2
9=r
The radius of the circle is equal to 9cm
Now we can solve for the area
A=πr²
A=π9²
A=π81
A=81πcm²
The exact area of a circle whose diameter is 18cm is 81πcm²
Answer=D