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An equation has a constant of proportionality if x is a multiple of y or vice-versa.

We simplify each equation

A) 4y=4x \\ y=x
B) 4y=12x \\ y=3x
C) 3y=4x \\ y= \frac{4}{3}x
D)3y=12x \\ y=4x

Only D has constant of proportionality equal to 4 so D is the answer