1) c) is perpendicular to the acceleration for one instant only. In general the velocity has two components, horizontal and vertical. Acceleration is vertical. So there is always some angle between them. But at the top most point of flight, the velocity is horizontal and acceleration is vertical. just at one instant
2) d) when displacement is increasing wrt time, its slope is velocity. Slope of a straight line is constant. after t0, the displacement is not changing. So position is not changing. so it is at rest.