Correct answer is (A). Natural monopoly occurs because of economies of scale.
Explanation: In case of monopoly, the average total cost (ATC) declines and gives monopolists the advantage over its competitors in the market. It is discrete type of monopoly where fixed cost are way too high which results in low entry and exit of new entrants and hence, natural monopoly. They comes with economies of scale, which means they create large relations to the size of market.