The correct answer is A. zero.
Marginal revenue is the additional revenue that a firm earns 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.
When a firm is producing at the quantity that generates maximum total revenue, marginal revenue is equal to zero. This is because the firm is earning the maximum amount of revenue for each additional unit sold.
If the firm produces more units than this, marginal revenue will be negative. This is because the firm will be earning less revenue for each additional unit sold.
Option B is incorrect because marginal revenue cannot be greater than 1. This is because the firm cannot earn more revenue for each additional unit sold than the price it charges for its product.
Option C is incorrect because marginal revenue cannot be less than -1. This is because the firm cannot lose money for each additional unit sold.
Option D is incorrect because marginal revenue cannot be infinite. This is because the firm cannot earn an infinite amount of revenue for each additional unit sold.