Concept:Percentage change in quantity supplied measures the change from the original quantity as a percentage.It is calculated using:% change=Original quantityNew quantity−Original quantity×100Explanation:Original quantity supplied = 400 units.New quantity supplied = 450 units.Increase in quantity = 450−400=50 units.Applying the formula:40050×100=12.5%Thus, the percentage change in quantity supplied is 12.5%.Answer:C. 12.5%