The correct answer is Calcium.
Milk fever is a condition seen mostly in dairy cows soon after calving. At this time, the body suddenly needs a large amount of calcium to produce milk, and if the blood calcium level drops too fast, the animal can develop weakness, low body temperature, and even collapse.
This condition is treated by giving the animal a calcium solution through injection. Farmers are also advised to manage the diet of pregnant animals so that calcium levels stay balanced before and after calving.