Correct Answer is D, Chef B is correct.
Broth should be kept at an even, gentle simmer.
When vegetables or meat is put on a stove with lots of water and is left on a low to medium flame, a broth is made. Broths are always kept at an even gentle simmer. They are never cooked on high flame. This is because of the fact that at a low flame, all the nutrients of the meat or vegetables come out and add proteins and vitamins in the soup. So it is always recommended to cook broth on a low flame.