A. Accelleration
A graphs slope has the formular rise/run, in this case, the change in speed/change in time (because speed is on the y-axis which means it is the "rise" and time is on the x-axis which means it is the "run").
If you go to a formulae sheet, you will see that a = change in speed/change in time.
This is the same as the formular of the graph therefore the slope of a speed-time graph indicates accelleration