Which line has an undefined slope? x=18, 2y-6x=0, y=-9, y=x

We know that

slope m=(y2-y1)/(x2-x1)
then

 case a) x=18
m=(y2-y1)/(18-18)> is undefined, because the denominator is zero

case b) 2y-6x=0
2y=6x> y=3x
the slope m=3> m is defined

case c) y=9
m=(9-9)/(x2-x1)> m=0> m is defined

case d) y=x
m=1> m is defined

the answer is
x=18 has an undefined slope

Where’s the following prism