The correct answer is A) Diamonds.
The industry that has not been considered a natural monopoly in the last 30 years is diamonds.
In financial terms, a natural monopoly is present when the high cost of producing something is present and only one company can afford to do that in industry. The government allows these monopolies to exist due to the specific activities they do at high costs, but the government set a series of regulation to oversee the business. Industries such as raw materials extraction or new technology development are examples of natural monopolies.
