The correct option to complete the sentence "Walking down the street ...." is B. a car knocked the boy down. Let's break down why this is the best choice and analyze the other options. Explanation of the Correct Answer (Option B)

1. Contextual Fit: The phrase "Walking down the street" sets the scene for an action that is happening. The subject of the sentence is implied to be the boy, who is likely the one walking. Therefore, the action that follows should logically relate to the boy's activity.
2. Active Voice: Option B uses the active voice ("a car knocked the boy down"), which is direct and clear. It indicates that the car is the subject performing the action of knocking down the boy. This makes the sentence straightforward and easy to understand.
3. Clarity: The phrase "a car knocked the boy down" clearly conveys what happened without ambiguity. It tells us that the boy was walking and was struck by a car, which is a common scenario that readers can easily visualize.

Analysis of Other Options

• Option A: the boy was knocked down by a car
• Voice: This option uses the passive voice. While it is grammatically correct, it is less direct than option B. Passive constructions can sometimes make sentences feel less immediate or engaging.
• Clarity: Although it conveys the same basic information, it does not emphasize the action as strongly as the active voice in option B.
• Option C: the boy was knocked down by car
• Grammatical Error: This option contains a grammatical error. It should say "by a car" instead of "by car." The absence of the article "a" makes the sentence incorrect.
• Voice: Like option A, it is in the passive voice, which is less direct.
• Option D: a car collided with the boy
• Meaning: While this option is also correct in terms of grammar, the word "collided" implies a different kind of impact than "knocked down." "Collided" suggests a more forceful encounter, which may not accurately reflect the scenario of a boy simply being knocked down while walking.
• Clarity: This option is less straightforward in terms of the action being described. It could imply that the boy was not necessarily knocked down but rather just hit, which may not align with the intended meaning of the original sentence.

Summary of Key Points

• Correct Option: B. "a car knocked the boy down" is the best choice due to its clarity and use of active voice.
• Passive vs. Active Voice: Active voice (Option B) is generally more engaging and direct than passive voice (Options A and C).
• Grammatical Accuracy: Option C is incorrect due to the missing article "a."
• Nuance of Meaning: Option D, while grammatically correct, changes the nuance of the action and may not fit the intended meaning as well as Option B.

Revision Summary

• Choose active voice for clarity and engagement in writing.
• Ensure grammatical accuracy by using correct articles.
• Understand the nuances of word choice to convey the intended meaning.
• Analyze all options critically to determine the best fit for the context.