The correct answer is: A. the same firm.
Multiple costing is a technique of using two or more costing methods for ascertainment of cost by the same firm. This is done to get a more accurate picture of the costs of production. For example, a firm might use job costing for some products and process costing for others. This would allow the firm to see the costs of each product in more detail.
B. several firms is incorrect because multiple costing is done by a single firm.
C. the same industry is incorrect because multiple costing can be used by firms in any industry.
D. the several industries is incorrect because multiple costing is done by a single firm, not by several industries.