Answer:
a vertical line.
Explanation:
A horizontal line is a completely flat line, a left-and-right line like the horizon. It has zero slope.
Perpendicular means that the two lines make right angles. They are at 90° to each other, like a floor and the wall.
A vertical line is a straight up-and-down line. It is perpendicular to the horizontal line.
A vertical line has slope that is undefined.