The correct answer is: A. a decrease in the number of units produced.
Marginal costing is a method of accounting that focuses on the costs that are directly related to the production of a product. These costs are known as variable costs. Variable costs change in proportion to the number of units produced. For example, the cost of raw materials is a variable cost because it increases as more units are produced.
Fixed costs are costs that do not change in proportion to the number of units produced. These costs include rent, insurance, and depreciation. Fixed costs are allocated to each unit produced using a cost allocation method such as direct costing or absorption costing.
Under marginal costing, unit product cost is calculated by dividing total variable costs by the number of units produced. Therefore, unit product cost would be increased by a decrease in the number of units produced because variable costs would be spread over a smaller number of units.
Option B is incorrect because an increase in the number of units produced would decrease unit product cost. This is because variable costs would be spread over a larger number of units.
Option C is incorrect because an increase in the commission paid to salesman for each unit sold would increase variable costs. This would increase unit product cost.
Option D is incorrect because a decrease in the commission paid to salesman for each unit sold would decrease variable costs. This would decrease unit product cost.