Choice "D" is correct. The difference between variable costing and full absorption costing lies in the treatment of fixed manufacturing costs. Full absorption costing treats fixed manufacturing costs as product costs, while variable costing expenses these as period costs. Using full absorption costing: $180,000 / 100,000 units$1.80 per unit produced $1.80 x 80,000 units sold$144,000 fixed manufacturing costs expensed (through cost of goods sold) under full absorption costing. The remaining fixed manufacturing costs of $36,000 ($180,000 - $144,000) remain in inventory as product costs. Variable costing treats all fixed costs as period costs, expensing the costs regardless of sales. Thus, the difference in net income would be the amount of fixed manufacturing costs inventoried under absorption costing: $180,000 - $144,000$36,000. Choice "b" is incorrect. Under full absorption costing, all manufacturing costs are product costs. Selling costs, regardless if fixed or variable, are all considered period costs.Choice "c" is incorrect. Under full absorption costing, all manufacturing costs are product costs. Selling costs, regardless if fixed or variable, are all considered period costs. Under variable costing, all fixed costs, including fixed manufacturing costs, are period costs.Choice "a" is incorrect. Under full absorption costing, all manufacturing costs are product costs. Selling costs, regardless if fixed or variable, are all considered period costs. Under variable costing, all fixed costs, including fixed manufacturing costs, are period costs.