Constant of proportionality
We know that the constant of proportionality shows us how much is the change of a variable in relation to the change of another variable.
In this case we want to see how much y changes when x change. In order to find it out we just have to divide them:

We have that the line cross the point (10, 55) from the origin (0, 0). Since (x, y), the change of x is 10 and the change of y is 55. Using the division, we have that:

is the constant of proportionality of this line.
Answer - A: 5.5