Which of the following line segments on a velocity versus time graph is physically impossible

You haven't listed any choices or offered any suggested answers. 
So there's no way for us to guide you to select the correct one, and
if I want to answer this question and scoop up a few points, I have to
invent an answer from scratch.

On a speed/time graph, a VERTICAL line segment represents something
that's physically impossible. 

The top of the line segment is a speed that's faster than the bottom
of the segment, but the times are both the same. 

A change of speed that takes no time would correspond to infinite
acceleration.  That just simply sounds wrong, and it is.  Can't happen.