Which of the following pairs of coordinates are on a line with an undefined slope? a. (2, 3) (9, 8) b. (3, 1) (4, 1 ) c. (2, 3) (2, 10)

The slope is defined as

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.