The correct answer is A. 0.4.
The degree of monopoly power is a measure of how much a monopolist can raise prices above marginal cost without losing all of its customers. It is calculated as the ratio of price to marginal cost, or P/MC. In this case, P = 10 and MC = 6, so the degree of monopoly power is 10/6 = 0.4.
Option B is incorrect because it is the price, not the degree of monopoly power. Option C is incorrect because it is the marginal cost, not the degree of monopoly power. Option D is incorrect because it is the inverse of the degree of monopoly power.
The degree of monopoly power is a useful measure of the market power of a firm. It can be used to compare the market power of different firms in the same industry, or to track changes in the market power of a firm over time.