In general,economists are critical of monopoly where there is/are:

A) no natural monopoly.
B) a natural monopoly.
C) only a few firms.
D) persistent economies of scale.

Economists are critical of monopoly where there is no natural monopoly, as this suggests that the monopoly is able to engage in anti-competitive behavior to maintain its market power, rather than simply benefiting from efficiencies of scale or other natural market forces. In contrast, a natural monopoly may be more acceptable, as it may be the most efficient way to provide a good or service due to high fixed costs or other factors. Having only a few firms in a market or persistent economies of scale may also raise concerns, but these factors alone do not necessarily indicate an anti-competitive monopoly.