A graph shows distance on the vertical axis and time on the horizontal axis. If the speed is steadily increasing, what will the line representing speed look like on the graph?

Wait just a minute.  You said it's a distance/time graph, so there's no line on it that represents speed.  The line represents distance.If the speed is steadily increasing, then as time goes on, more and more distance will be covered during each unit of time.  The distance line on the graph will curve upward, more and more steeply as time goes on.  If you put a ruler against the curve, then as time goes on, the slope of the ruler gets bigger and bigger, because the slope at any point on a distance/time graph is the speed at that time.