Answer: Option C
The sentence "While walking across the road a bus knocked him down" has a grammatical error. The subject and the verb are not in agreement, which makes the sentence unclear. To make the sentence grammatically correct, we need to rephrase it using the passive voice.
The correct answer is option C: "he was knocked down by a bus."
Let's see why the other options are incorrect:
Option A: "a bus knocked down" - This option is incomplete and does not provide any information about who was knocked down.
Option B: "he was by a bus knocked" - This option has a different word order that makes the sentence difficult to understand. It also uses the preposition "by" in the wrong position.
Option D: "knocked down by a bus" - This option is also correct, but it is incomplete. It does not provide any information about the person who was knocked down.
Option E: "No improvement" - This option means that the sentence is already correct, but as we have seen, it is not.
Now, let's take a closer look at option C, the correct answer. This option uses the passive voice, which means that the focus is on the action (being knocked down) rather than the person who performed the action (the bus).
The sentence "He was knocked down by a bus" is grammatically correct and clearly conveys the meaning of the original sentence. It tells us that someone (he) was knocked down by a bus while walking across the road.
In summary, to make the sentence grammatically correct, we need to rephrase it in the passive voice using the correct word order. The correct option is C: "he was knocked down by a bus."