When a graph between two physical quantities is a straight line, the two quantities are 

As we know the equation of a straight line is $Y=mX+C$

Let us assume $X$ and $Y$ are two different physical quantity and C is any constant. and the graph between them is straight line.

 According to above equation,    

                                           $Y\propto X$

Hence,  $X$ and  $Y$ are Directly proportional.

Option C