Final answer:
The constant of proportionality is a factor that relates two variables in direct proportion but cannot be determined without specific information or a given relationship. Therefore, the answer is 'Not enough information provided'(option d).
Step-by-step explanation:
The constant of proportionality is the factor by which a given variable must be multiplied to get another variable that is proportional to it. In the context of direct proportionality, represented by the formula 'y = kx,' k is the constant of proportionality. If y changes by 2 when x changes by 1, then k would be 2.
However, without specific numbers or a relationship given in the question, there's not enough information to determine the constant of proportionality. This concept is crucial in understanding how variables interact in equations and how changes in one affect another.