An equation is said to have a constant of proportionality if 'x' is a multiple of 'y' or 'y' is a multiple of 'x'.
Now, we have to determine an equation with constant of proportionality as '1'.
1.
This equation has constant of proportionality as ''.
2.
This equation has constant of proportionality as ''.
3.
This equation has constant of proportionality as ''.
4.
This equation has constant of proportionality as '1'.
Option D is the correct answer.