I think the answer would have to be D. If the diameter, which is the distance from one point on the circle through the center to another point on the circle, is halved then the amount of concrete that will be needed will be multiplied by 4. I think that's right I am so sorry in advance if that was incorrect. Have a beautiful rest of your day. Stay safe. C O V I D - 1 9 IS REAL THIS COULD MEAN YOUR LIFE!!!