Final answer:
Two quantities x and y that are always a constant multiple of one another display Direct proportionality, characterized by the equation y = kx, where k is a constant, and their relationship forms a straight line through the origin on a graph.
Step-by-step explanation:
For two quantities x and y, if y is always a constant multiple of x, then they have a Direct proportionality. This means that as x increases or decreases, y will also increase or decrease respectively in such a way that the ratio of y to x remains constant. The relationship between x and y can be represented by the equation y = kx, where k is the proportionality constant. When plotting this relationship on a graph, you would see a straight line that passes through the origin (0, 0). This is because in direct proportion, the change in x is always matched by a proportional change in y.