In Which of the Following Situations Would a Natural Monopoly

In which of the following situations would a natural monopoly exist?

A) a firm has a government charter to be the sole producer of some good
B) a firm is able to operate at the minimum point of its long-run average total cost curve
C) a firm produces a product essential to national security
D) only one firm is supplying a natural resource
E) one firm can most efficiently supply the entire market demand