Question

Which of the following pairs of coordinates are on a line with an undefined slope?

Answer 1

The slope is defined as

$m = \dfrac{\Delta y}{\Delta x}$

which means that you have the difference of the y coordinates of the two points at the numerator, and the difference of the x coordinates of the two points at the denominator.

So, if the two points have the same x coordinate, the denominator vanished, and the slope is not defined.

Since the only points with the same x coordinate are those in option c, that is the correct answer.