Economies of scale occur
a. when the marginal cost of production decreases with output.
b. when the marginal product of labor increases with output.
c. when a​ firm's long-run average costs increase with output.
d. when a​ firm's long-run average costs decrease with output.

A. when the marginal cost of production decreases with output.

Explanation:

the marginal cost is the cost of producing an additional unit.

In economies  of scale it will decrease when an increase in production occurs. As the company can negociate lower cost and spread over his fixed cost iver more units of productions. These situation makes the company become more efficient as well thus, lower their cost as a result.