Final answer:
Example (a) Y = 2x is the correct representation of the equation Y = kx, where k is a constant that defines the direct proportional relationship between Y and x.
Step-by-step explanation:
From the equation Y = kx, example (a) Y = 2x correctly represents an equation where k is a constant multiplier of the independent variable x, resulting in the dependent variable Y. Here, the constant k is 2, which means for each unit increase in x, Y will increase by 2 units. This relationship is a direct variation, and it creates a straight line graph when plotted, demonstrating that Y is directly proportional to x.