Sentence given below has been divided into four parts. One of the parts…

Answer: The part containing the main error is "Walking".

Explanation: "Walking" is a participle that introduces a participle phrase. As placed at the start, it creates a dangling participle because there is no appropriate subject performing the action of walking. The sentence as written implies the truck was walking. The correct structure clearly links the walking action to the person.

• Corrected full sentence: "While he was walking across the street, a truck knocked him down."

• Also fix the typographical error in the second part: change "across the streett" to "across the street."

Why other parts are not the primary error:

• "a truck" is correct because it names the agent that caused the action.

• "knocked him down" is grammatically correct as the main verb phrase describing the event.