What Is the Definition of a Natural Monopoly

What is the definition of a natural monopoly?

A) an industry in which one firm is very large relative to other firms that could enter the industry
B) an industry in which a single firm controls crucial inputs to the production process
C) an industry in which a single seller exists as a result of patent protection
D) an industry in which one firm can produce the entire industry output at a lower average cost than can a larger number of firms