Well lets look at the definitions and formulas of both to compare.
The formula of Constant of Variation:
Since k is constant (the same for every point), we can find k when given any point by dividing the y-coordinate by the x-coordinate. For example, if y varies directly as x , and y = 6 when x = 2 , the constant of variation is k = = 3 . Thus, the equation describing this direct variation is y = 3x .
The formula of Constant of Proportionality:
The constant is called the coefficient of proportionality orproportionality constant. If one variable is always the product of the other and a constant, the two are said to be directly proportional. x and y are directly proportional if the ratio yx is constant.
Compare them and you'll see the answer,
I hope i helped, and tag me if you need anything :)
Thank You,
Darian