The correct answer is: A. Perfect competition.
In perfect competition, there are many buyers and sellers of a homogeneous good, and no one buyer or seller has any market power. This means that the demand for a firm’s product is perfectly elastic, and the firm can sell as much or as little as it wants at the market price.
In monopoly, there is only one seller of a good or service. This means that the monopolist has a great deal of market power, and can charge a price that is above the competitive price. The demand for a monopolist’s product is less elastic than the demand for a firm in perfect competition.
In imperfect competition, there are a few sellers of a good or service. This means that each seller has some market power, but not as much as a monopolist. The demand for a firm in imperfect competition is less elastic than the demand for a firm in perfect competition, but more elastic than the demand for a firm in monopoly.
In oligopoly, there are a few sellers of a good or service, and the products of the sellers are differentiated. This means that each seller has some market power, and the demand for each seller’s product is less elastic than the demand for a firm in perfect competition. However, the demand for each seller’s product is not as inelastic as the demand for a firm in monopoly.
In conclusion, the demand for a commodity is completely elastic in perfect competition. This is because there are many buyers and sellers of a homogeneous good, and no one buyer or seller has any market power.