SAT Question 0837c3b9: Area and Volume answer and explanation with Desmos solution

Choice B is correct. Since triangles $ABC$ and $DEF$ are similar and the length of $\overline{DE}$ is $2$ times the length of $\overline{AB}$, the length of each side of triangle $DEF$ is two times the length of its corresponding side in triangle $ABC$. Therefore, the perimeter of triangle $DEF$ is two times the perimeter of triangle $ABC$. Since the perimeter of triangle $ABC$ is $20$, the perimeter of triangle $DEF$ is $40$.