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.