What is the contrapositive of the conditional statement?

If two variables are directly proportional, then their graph is a linear function.

If two variables are directly proportional, then their graph is not a linear function.
If two variables are not directly proportional, then their graph is not a linear function.
If the graph of two variables is a linear function, then the two variables are directly proportional.
If the graph of two variables is not a linear function, then the two variables are not directly proportional.