The correct answer is C. MR and MC.
A monopolist is the only seller of a good or service in a market. This means that they have a great deal of market power and can set the price of their product. In order to maximize profit, a monopolist must equate marginal revenue (MR) and marginal cost (MC).
Marginal revenue is the additional revenue that a firm earns by selling one more unit of output. Marginal cost is the additional cost that a firm incurs by producing one more unit of output. When MR = MC, the firm is producing at the point where the additional revenue from selling one more unit of output is equal to the additional cost of producing that unit. This is the point at which the firm is maximizing its profit.
The other options are incorrect because they do not represent the optimal production level for a monopolist. Option A, AR and MC, is incorrect because average revenue (AR) is not equal to marginal revenue. Option B, AR and MR, is incorrect because average revenue is not equal to marginal cost. Option D, TR and MC, is incorrect because total revenue (TR) is not equal to marginal cost.
In conclusion, the correct answer is C. MR and MC.