Final answer:
The constant of proportionality (r) in the equation y=rx cannot be determined without a specific pair of values for y and x. To find r, divide a given value of y by the corresponding value of x.
Step-by-step explanation:
The constant of proportionality, often represented by r, in the equation y = rx, defines the ratio of y to x. In the context of this equation, r is a numerical value that shows the amount by which y changes when x changes. Unfortunately, without additional information or data points that give specific values of y and x, we cannot determine the exact value of r from the given options, as the equation is simply a generic representation of a direct proportion. To find r, we need at least one pair of corresponding values for y and x. Once we have that pair, we could divide the value of y by the value of x to find the constant of proportionality.