Answer: Choice C) k = 6
One way to find the constant of proportionality is to locate the point of the form (1,k). We see that (1,6) is on the line so k = 6.
Another way to get the constant of proportionality is to compute k = y/x where x is not zero. The (x,y) point must be on the diagonal line.
So we could pick say (x,y) = (2,12) to get k = y/x = 12/2 = 6 which is the same value as before.
The equation of this line is y = 6x. So the constant of proportionality is the slope of the line. Direct proportion equations always go through the origin.