The correct answer is B. marginal revenues from the separate market.
A multi-market seller is a firm that sells its products in multiple markets. The rule for optimization for a multi-market seller is to equate marginal revenue in each market to marginal cost. This means that the firm should produce in each market until the additional revenue generated by selling one more unit is equal to the additional cost of producing that unit.
Marginal revenue is the additional revenue that a firm generates by selling one more unit of its product. It is calculated by taking the change in total revenue and dividing it by the change in quantity sold. Marginal cost is the additional cost that a firm incurs by producing one more unit of its product. It is calculated by taking the change in total cost and dividing it by the change in quantity produced.
When a firm equates marginal revenue to marginal cost, it is maximizing its profits. This is because the firm is producing the quantity of output at which the additional revenue generated by selling one more unit is just equal to the additional cost of producing that unit. Any additional output would generate less revenue than it costs to produce, and any less output would generate more revenue than it costs to produce.
The other options are incorrect because they do not represent the rule for optimization for a multi-market seller. Option A, average revenues from the separate market, is not a relevant concept for optimization. Option C, total revenue from the separate market, is not a relevant concept for optimization because it does not take into account the cost of production. Option D, marginal price of production for separate markets, is not a relevant concept for optimization because it does not take into account the revenue generated by selling the product.