Final answer:
In a proportional relationship, the equation that models it is y = kx.
Step-by-step explanation:
In a proportional relationship, two variables, x and y, are directly proportional if a change in one variable leads to the same change in the other variable. The equation that models a proportional relationship is y = kx, where k is the proportionality constant. This equation represents a straight line graph that passes through the origin (0,0).