The correct answer is: B. control over supply.
A monopoly is a market structure in which there is only one seller of a good or service. This means that the monopolist has a great deal of control over the price of the good or service, as well as the quantity that is produced.
The monopolist can control the supply of the good or service by choosing how much to produce. If the monopolist produces a small quantity of the good or service, the price will be high. If the monopolist produces a large quantity of the good or service, the price will be low.
The monopolist can also control the supply of the good or service by choosing how to price it. If the monopolist prices the good or service high, consumers will buy less of it. If the monopolist prices the good or service low, consumers will buy more of it.
The monopolist’s control over supply gives it a great deal of power in the market. The monopolist can use this power to charge high prices and earn high profits.
The other options are incorrect because they do not accurately describe the basis of a monopoly.
Option A is incorrect because demand is determined by the preferences of consumers. The monopolist cannot control the preferences of consumers.
Option C is incorrect because the monopolist does not have control over the population. The population is determined by a number of factors, including birth rates, death rates, and immigration rates.
Option D is incorrect because the monopolist does not have control over the price level. The price level is determined by a number of factors, including the supply of money, the demand for goods and services, and the level of economic activity.