Final answer:
The supply curve for a region's lumber industry is the horizontal summation of each lumber firm's marginal cost curves, which serve as their supply curves.
Step-by-step explanation:
The supply curve for the entire lumber industry in a region is indeed the horizontal summation of the marginal cost curves for each lumber firm in the region when we assume perfect competition. That's because each firm's marginal cost curve, above its average variable cost curve, represents its supply curve. As firms receive prices from the market, they decide how much to supply based on where the price equals their marginal cost.
Now, if the marginal costs of a firm increase, the firm's supply curve would shift upwards. This upward shift indicates that at each price level, the quantity supplied by the firm would decrease. For the lumber industry, the market supply curve would shift to the left, resulting in a decrease in the quantity supplied at each price, assuming all else is equal.
The correct answer to the student's question is: a. Is the horizontal summation of marginal cost curves for each lumber firm in the region.