Concept:A preposition is chosen based on the sentence that contains the blank.
Explanation:The given question lists only the options: A. by, B. at, C. in, D. for.
The actual sentence to complete is missing from the question paper.
Without that sentence, we cannot know the required relationship between words.
Each preposition could be correct in a different sentence.
Therefore, no single option can be verified as the best answer.
Answer:The question is incomplete, so the correct option cannot be determined.