Given the equation y/x = 2/3, the constant of variation is
a)2
b)2/3
c)3

This is an example of what's called a direct variation, meaning that y/x always = the same thing. In this case, that would be 2/3, so the answer to your question is b.

You can rearrange the equation this way. (multiply both sides by X)

y=\frac 23 x

In this form you can see that Y varies with X, and the proportionality constant is the coefficient of X (2/3).