Which Sentence must be revised to eliminate the Preposition at the End?

Which sentence must be revised to eliminate the preposition at the end?

A. If no one answers the door, then please let yourself in.

B. The top steps are fine, but be careful of the slippery stairs below.

C. Where are Katelyn and Ella walking to?

D. Do you know what Naveen was talking about?
 

Answer: C. Where are Katelyn and Ella walking to?
 

Explanation:

Sentence C  must be revised to eliminate the preposition at the end. Formal written English overall discourages ending sentences with prepositions except that rewriting leads to unnatural or unclear wording. Sentence C ends with the preposition "to," and this can be rewritten so that it is clearer and more specific. A rewritten sentence would be: "To where are Katelyn and Ella going?" While free conversation permits ending with prepositions, academic or professional writing overall prefers more formal phrasing.
 

Sentences A, B, and D are correct, with prepositions in position in the sentence and not require alteration. For example, "talking about" (D) is a phrasal verb, and altering it would sound unnatural (e.g., "About what was Naveen talking?" feels overly formal and stilted).