A natural monopoly occurs when:

• A. firms collude to set prices above the competitive level
• B. there are many firms in the market, each with identical cost structures
• C. a single firm can supply the entire market at a lower cost than multiple firms could
• D. government regulation forces all firms to merge into one

Answer: (C)

Verified step by step guidance

1. Understand the definition of a natural monopoly: it occurs when a single firm can supply the entire market demand at a lower cost than multiple firms could, due to economies of scale over the relevant range of output.
2. Recognize that this situation arises because the firm's average total cost (ATC) decreases as output increases, making it inefficient for multiple firms to operate simultaneously.
3. Contrast this with other options: collusion involves firms cooperating to set prices, which is different from a natural monopoly; many firms with identical costs describe a competitive market, not a monopoly; government regulation forcing mergers is a policy action, not the cause of a natural monopoly.
4. Focus on the cost structure and market demand relationship: the key characteristic is that the cost advantage of one firm serving the whole market leads to a natural monopoly.
5. Summarize that the correct understanding is that a natural monopoly exists when a single firm can produce the total market output at a lower cost than any combination of multiple firms.