In (a) velocity and acceleration are mutually perpendicular, so velocity remains constant. In (b), if velocity is in the direction of acceleration (or of the same sign as that of acceleration) then speed increases. And if velocity is in opposite direction, then speed decreases.In (c) velocity and acceleration areoppositely directed,so velocity is decreasing  In (d) x component of velocitry is decreasing. Therefore, speed is decreasing..