(a) Correct. Example, if the ball is in vertically upward motion, i.e., throw vertically upward, then it will be of zero speed at the highest point and an acceleration of 9.8 ms-2 in downward direction.
(b) Incorrect, because speed is the magnitude of velocity.
© Correct, if the particle rebounds instantly with the same speed acceleration will be zero.
Example, Elastic collision of a ball with an elastic floor.
(d) Wrong. Example, if we take the positive direction
of acceleration due to gravity downwards, a ball thrown vertically upwards will have its speed decreasing. However, the statement can be correct if the positive direction of acceleration is along the direction of motion.