constant acceleration- a
constant velocity- c
no velocity or acceleration- b
explanation:
for body moving with a constant acceleration, the rate of change of displacement changes over time. this is proven by the increasing gradient of the curve (a) against time.
a body that is moving at constant velocity covers equal distances over equal periods of time, thus the straight line with a constant gradient c.
a stationary body covers no displacement as time elapses. thus the flat line with zero gradient.