Don't judge the boy … his past.

Analyze the question: The sentence is "Don't judge the boy __ his past." We need the preposition that correctly follows the verb judge in this meaning.

Explain the concept: The verb judge often takes the preposition by when we mean "to form an opinion about someone based on something." For example:

• "Don't judge a book by its cover."
• "People should be judged by their actions, not their appearance."

Here, by shows the standard or basis used to form the opinion.

Check each option:

• A. from: "judge from" can be used in the sense of "judging from the evidence" (a phrase meaning "based on"), but it doesn't fit with a direct object like the boy.
• B. at: Not used with judge in this sense.
• C. by: Correct. "Judge the boy by his past" means to form an opinion about him based on his past.
• D. for: "judge for" appears in "judge for yourself," but it doesn't fit here. (Note that you can be judged for a specific offence, but "judge the boy for his past" is less idiomatic than "by" when the sense is "on the basis of.")

Answer: The correct option is **C.

Tip: Remember the fixed phrase "judge by" (judge by appearance, judge by results, judge by one's past). If you see judge followed by a blank in an exam and the sense is "on the basis of," choose by.