The correct answer is D. No improvement.
The sentence "While crossing the highway a five year old child was knocked out by a passing car" is grammatically correct and conveys the intended meaning clearly. The word "out" is used correctly to indicate that the child was unconscious after being hit by the car.
The other options are not necessary or improve the sentence. The word "away" is not needed, as the sentence already indicates that the child was hit by a car. The word "up" is not needed, as the sentence does not indicate that the child was lifted up by the car. The word "down" is not needed, as the sentence does not indicate that the child was knocked down by the car.