Answer: 3
Step-by-step explanation:
Given : x and y are two quantities.
The graph shows the amount of money paid when purchasing bags of caramel corn at the zoo represented by a line.
Two points through which graph is passing : (0,0) and (1,3)
Slope of graph : 

If a directly proportional relationship is represented by a line on a the graph, then the constant of proportionality is the slope of the line.
i.e. The constant of proportionality =3