Complete idiom: “Do not judge a book by …”

The idiom "Do not judge a book by its cover" is a common English expression that advises against forming an opinion or making a judgment about someone or something based solely on their outward appearance. The "cover" represents the superficial or external aspect, while the "book" symbolizes the true nature or content. This idiom encourages looking beyond initial impressions to understand the deeper qualities.

• Option B, "Content," is incorrect because the idiom specifically warns against judging based on superficiality, not the actual content, which is what one should evaluate.
• Options C, "Language," and D, "Title," are also aspects of a book's presentation, but "cover" is the universally recognized and correct word to complete this specific idiom, emphasizing external appearance.