Correct Option: A

The supply function expresses the relationship between the total quantity supplied and the price received by all suppliers per unit of time, holding other factors constant. It illustrates the relation between price and supply.
The diagram (Price is shown on the Y-axis and Quantity per day on the X-axis) shows that suppliers will produce quantity Q1 units of a good if the price they receive is P1. As the price keeps decreasing, the quantity produced also keeps on decreasing. So though the supply function has to do with supply and price, it can be perceived to express similar functional relationship between price and output (in terms of quantity that will be produced).