In economic efficiency, marginal benefit equals marginal cost at:

In the context of economic efficiency, the principle that marginal benefit equals marginal cost is crucial for determining the optimal level of production and consumption. When these two concepts are equal, it indicates that resources are being allocated in a manner that maximizes overall welfare or satisfaction.

At the last unit sold, the benefit derived from consuming that unit matches the cost incurred to produce it. This balance ensures that no further gains can be achieved by producing additional units; any extra production would result in a marginal cost that exceeds the marginal benefit, leading to a reduction in overall efficiency. Therefore, operating at this point means that the economy is using its resources effectively, without excess or scarcity.

The other choices reflect points in the consumption or production process that do not align with this principle. For instance, the first unit sold may not capture the total marginal costs involved in production, while stating "any unit sold" fails to recognize that efficiency is found specifically when the benefit is balanced with the cost at the last unit. Lastly, asserting "none of the units sold" overlooks the fact that economic efficiency is achievable at a specific point where marginal benefit and marginal cost converge, which occurs at the last unit sold.