The part labeled "C" in a graph is the "equilibrium point". This is where the quantity of a good or service that consumers are willing and able to buy (i.e. the quantity demanded) is equal to the quantity that producers are willing and able to sell (i.e. the quantity supplied). In other words, it's the point where there is balance between the amount of a product that is being bought and sold, and there is no surplus or shortage.
The part labelled C in the graph is the?