Answer: C. economies of scale.
Explanation: The analysis is a concept of economies of scale. It's is the characteristics of a production process in which an increase in the scale of the firm (expanding the output of its biggest US plant by 20%) causes a decrease in the long run average cost of each unit (decrease from $67.50 to $42.5 per unit cost).
The size of the company matters when it comes to economies of scale (cost advantages reaped by companies when production processes becomes efficient) because the larger the business, the more the cost savings.