How many milligrams are equivalent to one kilogram?

One kilogram is equivalent to 1,000 grams, as the metric system is based on powers of ten. To convert grams into milligrams, you need to realize that one gram equals 1,000 milligrams. Therefore, when converting one kilogram to milligrams, you multiply the number of grams (1,000) by the number of milligrams in each gram (1,000). This leads to:

1 kg = 1,000 g × 1,000 mg/g = 1,000,000 mg.

Thus, one kilogram is equal to one million milligrams, confirming that the correct answer of D is based on the direct application of the metric system's unit conversion toward larger and smaller measures.