B because when you take the square root of a negative number, it is imaginary (i). That eliminates A,C,D. When you take the square root of -216, the radical will no longer be negative since you identified it as imaginary. The greatest perfect square that you can take from 216 is 36.( 36 x 6= 216). So the square root of 36 is 6. So 6i radical 6 which is B. You can’t simplify the 6 inside the square root because it is not a perfect square. Hope this helps!
