The correct answer is: A. Ratio between price and marginal cost
According to M. Kalecki, the true measure of the degree of monopoly power is the ratio between price and marginal cost. This is because the higher the ratio, the more the monopolist is able to charge a price above marginal cost, and the greater the monopolist’s profits will be.
Option B is incorrect because the extent of monopolistic profit enjoyed by the monopolist is not a direct measure of the degree of monopoly power. A monopolist can enjoy high profits even if it has a low degree of monopoly power, if its costs are low.
Option C is incorrect because the cross-elasticity of demand for the product of the monopolist is a measure of the responsiveness of demand for the monopolist’s product to changes in the price of a substitute product. It is not a direct measure of the degree of monopoly power.
Option D is incorrect because the price charged by the monopolist minus marginal cost of production is a measure of the monopolist’s profit margin. It is not a direct measure of the degree of monopoly power.