Step 1: Understanding the Concept:
This is a contextual vocabulary question (Fill in the blanks).
To solve this, one must perform a "clue search" within the sentence to identify the logical tone required.
The phrase "so \_\_\_\_\_\_\_\_ that even his opponents were moved" contains the result of the action.
"Moved" in this context means emotionally affected, touched, or persuaded.
The word "even" suggests that the speech was so powerful that it broke through the natural resistance or bias of the "opponents."
Step 2: Detailed Explanation:
We must now evaluate each adjective to see which one logically results in "moving" an audience, especially a hostile one.
1. Analyzing the Options:
(D) Eloquent: This is the correct choice. 'Eloquent' describes someone who is fluent, forceful, and persuasive in speaking. An eloquent speech is characterized by its beauty and its ability to stir the emotions of the listeners. This is exactly the kind of speech that would move an opponent.
(A) Banal: This refers to something so lacking in originality as to be obvious and boring. A banal speech consists of clichés and uninspired ideas. It would likely cause boredom or annoyance, not emotional movement.
(B) Monotonous: This describes a sound or speech that is dull and repetitive, lacking in variety or interest. A monotonous voice usually makes listeners lose focus or fall asleep.
(C) Garbled: This means a message that is distorted or confused to the point of being unclear. If a politician's speech was garbled, the opponents wouldn't even be able to understand the message, making it impossible for them to be "moved" by it.
2. Sentence Logic (Cause and Effect):
The sentence follows the structure: Cause (the quality of the speech) $\rightarrow$ Effect (opponents being moved).
Only 'eloquent' provides a positive, powerful cause sufficient to create the described effect.
Example: "Dr. Martin Luther King Jr. was known for his eloquent speeches that changed the course of history."
Step 3: Final Answer:
Because "eloquent" describes a persuasive and moving style of speech, it is the only word that fits the context of influencing opponents. Thus, Option (D) is correct.