Concept:Mark-up is always calculated on the cost of sales, not on the selling price.Explanation:Let the cost of sales be C.Gross profit mark-up is 25% of cost, so gross profit =0.25C.Sales = Cost of Sales + Gross Profit.Therefore, D12,000=C+0.25C=1.25C.Solving for C: C=1.25D12,000.C=D9,600.Thus, the cost of sales is D9,600.Answer:C. D9,600