The correct answer is: A. per-unit production tax.
A per-unit production tax is a tax that is levied on each unit of a good that is produced. This type of tax can be used to reduce the output of a monopoly firm and increase market efficiency.
A monopoly firm is a firm that is the only seller of a good or service in a market. This means that the firm has a great deal of market power and can charge a price that is higher than the competitive price. This results in a deadweight loss, which is a loss of economic efficiency.
A per-unit production tax can reduce the output of a monopoly firm by increasing the cost of production. This will cause the firm to produce less output and charge a lower price. This will result in a decrease in the deadweight loss and an increase in market efficiency.
The other options are incorrect because they would not reduce the output of a monopoly firm and would not increase market efficiency.
A per-unit sales tax is a tax that is levied on each unit of a good that is sold. This type of tax would not affect the cost of production for a monopoly firm and would not cause the firm to produce less output.
A profit tax is a tax that is levied on the profits of a firm. This type of tax would not affect the cost of production for a monopoly firm and would not cause the firm to produce less output.
A price ceiling below the existing equilibrium price would cause a shortage of the good, which would not increase market efficiency.