The correct answer is: A. positive.
Marginal revenue is the additional revenue that a firm earns from 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.
The elasticity of demand is a measure of how responsive consumers are to changes in price. It is calculated by taking the percentage change in quantity demanded and dividing it by the percentage change in price.
When the elasticity of demand is equal to one, then consumers are perfectly responsive to changes in price. This means that if the price of a product goes up by 1%, then the quantity demanded will go down by 1%.
In this case, the marginal revenue will be positive. This is because the firm will earn more revenue from selling one more unit of its product, even though the price of the product has gone up.
The other options are incorrect because:
- Option B: negative. This is not possible when the elasticity of demand is equal to one.
- Option C: zero. This is only possible when the elasticity of demand is infinite.
- Option D: infinity. This is not possible.