The area of the circle shown below is 100π. What is the diameter (D) of the circle?

[tex]\bf \textit{area of a circle}\\\\ A=\pi r^2~~ \begin{cases} r=radius\\ -----\\ A=100\pi \end{cases}\implies 100\pi =\pi r^2 \\\\\\ \cfrac{100\pi }{\pi }=r^2\implies 100=r^2\implies \sqrt{100}=r\implies \boxed{10=r}[/tex]

since a diameter is twice as long as the radius, this one is also 2r.