Final answer:
To identify the incorrect statement about the firm's cost structure, one calculates the average fixed cost by dividing the fixed costs by the output quantity. The provided information accurately describes the firm's average fixed cost given constant fixed costs and a set output level.
Step-by-step explanation:
The student's question pertains to microeconomics and involves understanding concepts such as marginal cost, economies of scale, average fixed cost, and fixed costs. Assessing the statements provided, we see that the firm is described in several contrasting ways: (1) having constant marginal cost, (2) experiencing economies of scale, (3) an average fixed cost of $20 at 4 units of output, and (4) fixed costs of $80. To determine which of these statements is incorrect, we can recall that average fixed cost is calculated by dividing total fixed costs by the quantity of output. If the total fixed costs are $80 as stated in the fourth option, and there are 4 units of output, the average fixed cost would be $80 divided by 4, which is $20 per unit. Therefore, this statement is correct. Since the other options require additional information or are theoretical concepts, evaluating their correctness would depend on the specific characteristics of the firm's production which have not been provided.