A change in quantity supplied is a movement along the supply curve in response to a change in price, while a change in supply is a shift of the entire supply curve in response to something besides price.

From the question, the price was the same when the quantity supplied changed from 100 units to 120 units. This means the reason for the movement or shift is not because of a change in price, but due to other factors. Hence the correct option is option A. It means a change in supply is the reason for the movement.