Step 1: Understanding the Concept:
This question tests the accurate application of definite articles ("the") when designating specific titles, metaphors, or previously mentioned entities.
Step 2: Detailed Explanation:
1. Sentence (A): "The lion is king of beasts."
Incorrect. When a noun acts as a unique title, it requires "the." It should be "The lion is the king of beasts."
2. Sentence (B): "The rose is queen of flowers."
Incorrect. Similar to sentence A, it requires a definite article for the unique title: "The rose is the queen of flowers."
3. Sentence (C): "The pen is mightier than the sword."
Correct. This is a standard proverb where the definite article is correctly applied to general classes.
4. Sentence (D): "This is a boy I told you about."
Incorrect. The clause "I told you about" specifies exactly which boy is being discussed. Specificity requires the definite article: "This is the boy I told you about."
Step 3: Final Answer:
Options (A), (B), and (D) are grammatically incorrect due to missing definite articles.